As the garrison forces moved in, there was sometimes a scarcity of provisions and more than one famine occurred. Excessive drinking was a problem, but life for the merchants was good. Visitors were amazed that wealth and comfort could be amassed from a single industry, the fishery. Trade was carried on with New England, Quebec and the West Indies. Treasure ships from South America and the West Indies found Louisbourg a good spot to refit their ships.

However, this peace and comfort would not last. French privateers forayed into Canso and Annapolis Royal and provoked New England militiamen to retaliation. Governor William Shirley of Massachusetts ordered an attack on Louisbourg by forces led by William Pepperrell in 1745. Amazingly, the New England militia, supported by the British, was successful in their attack and the siege ended with Louisbourg's surrender in July. Many of the inhabitants were removed and militiamen and the regular British forces held the town until 1749 when under Article IX of the Treaty of Aix-la-Chapelle, Louisbourg and Cape Breton were given back to France.

After this siege, Louisbourg's importance became more apparent to Louis XV. He sent more garrison troops and engineers to the colony to strengthen the fortress. However, their efforts were unsuccessful. Hostilities broke out in 1756 between France and England in the Seven Years War and Louisbourg was taken again two years later by the British forces, under General James Wolfe.

With Louisbourg in British hands, the rest of New France was laid open to attack. This time the British made sure Louisbourg would never again be a threat to their interests and they demolished the town’s fortifications in 1760.

What remained of the once proud fortress was crumbling walls. The area grew over with grass and was eventually used as a pasture for neighboring farmers. The following 1849 description from Abraham Gesner, fellow of the Geological Society, describes what amateur historians like J.S. would have seen when viewing the site:

The lighthouse on the northern side of the entrance of the harbour stands on a bold rocky cliff, once occupied by a strong battery. The dilapidated walls of the great battery of guns on the northern side of the harbour, and another on the opposite shore, now appear like natural mounds, being covered with clover, and other grasses. The little island at the harbour’s mouth has yielded to the operation of the waves, and a part of the fortifications have fallen into the sea.... The arched places of arms, and bomb- proofs of the citadel, are still entire. Three of them are sheepfolds - another is occupied by a fisherman for a cabbage cellar, and time has encrusted the ceilings of all with small stalactites. The foundations of the barracks, chapels, nunnery, hospital, and other public buildings, are still perfect; and the cells of the prison are almost unbroken, as is also the kiln of the large brewery.⁹

This was the site with which J.S. became fascinated and he would contend that enough of the ruins remained to construct the town as it once was. While this seemed like an impossible dream to many, J.S. maintained his vision of the restoration as a possible reality. In a speech at Kennington Cove in August of 1931, J.S. said that he hoped Louisbourg would "be restored that one may see the form of the town, long ago developed by enterprise and endurance into a place of importance."¹⁰ In an even earlier speech in 1909, J.S. stated he was convinced that restoration was possible: "In short what I have seen of the site and of the documents I feel certain that the complete reconstruction of the place is only a question of intelligence and outlay."¹¹ This intelligence and outlay started with J.S. and Katharine, and led to the site's eventual restoration.

Oddly enough, the first actual step toward this process was made by a group from the United States. In 1895, the Colonial Wars Society of America erected a large monument in Louisbourg to honour the New England militia's capture of the fortress in 1745. This action was seen as insulting by many. The most outspoken critics were French Canadians whose newspapers decried the intention on the part of the Colonial Wars Society. Pascal Poirier, a French Canadian politician, was very vocal about this perceived insult. The United Empire Loyalists Association of Canada also protested the action. However, the monument was unveiled as scheduled by Lieutenant-Governor Sir Malachy Daly of Nova Scotia with 2500 people in attendance, including many U.S. and Canadian dignitaries.

The next player on the Louisbourg stage was Captain P.J. Kennelly, an industrialist who was attempting to modernize Cape Breton industry. He became interested in preserving Louisbourg and in early 1900 began purchasing lots in and around the ruins. In 1903, Kennelly formed the Louisbourg Memorial Fund to finance the work of preservation. This work would include the stabilization of the foundations, the fencing off and improvement of burial grounds, and the erection of a large masonry tower to be used as a museum. Kennelly was successful in getting support for his project. King Edward VII was the honourary patron of the Fund and vice-patrons included former Prime Minister Sir Charles Tupper, future Prime Minister Sir Robert Borden, six provincial lieutenant-governors and five American state governors. Kennelly personally oversaw preservation of the site. A great step toward preservation came in 1906. The Nova Scotian legislature passed an Act to Incorporate the Trustees of the French Fortress and Old Burying Grounds at Louisbourg as a Historical Monument of the Dominion of Canada and as a Public Work. The act named eleven people trustees (one British, one American and the rest Canadians) of the site. The trustees had the power to acquire more land for preservation. However, in 1907, Kennelly died and preservation came to a standstill. Here J.S. picked up the reins and began his appeals to the government and the Historical Society for reconstruction of the Intendent's House, which he believed could serve as a museum.

Until this point, no government body was formally responsible for historic preservation. The Historic Sites and Monuments Board of Canada (HSMBC) was established in 1919 and Louisbourg was the first site they examined. The HSMBC found that the land in question was made up of properties owned by the Cape Breton Railway Co., the Kennelly estate, and two dozen local families. In 1921 it was suggested that a caretaker be hired for the site to keep people from removing relics. Also in that year, sixty-nine acres were acquired from the Cape Breton Railway Company by the Department of the Interior. A special sub-committee of the HSMBC was formed of Dr. J.C. Webster and Mayor J. Plimsoll Edwards of Louisbourg to advise the Board. They suggested, after a 1923 visit, that all of the land of the Old Town, including the lighthouse and batteries, be purchased, and an on-site and permanent caretaker should be hired. They also recommended that an engineer be hired to map out the streets, buildings, and fortifications. This report was supported by J.S. and Archdeacon Draper, Anglican Minister and friend of J.S. at Louisbourg. The HSMBC concluded that this would be impossible without Parliamentary appropriation of funds. Instead, four memorial cairns were erected at Louisbourg. However, in 1924, Webster extracted a promise from Prime Minister Arthur Meighen to lobby for more funding. Later that year the Department of the Interior's Parks Branch finally received title to the Kennelly lands. Now the government possessed 70 acres, or 1/5 of the recommended land. Webster, Draper, and McLennan continued to press for more development. In 1928, the federal government appropriated $19,000 to purchase most of the privately owned land and another $3,000 for development. This year also saw the ruins being declared a National Historic Site.

At this time, two differing opinions emerged about what should be done at the site. One contended that the ruins should be left alone. However, McLennan and Webster were "reconstructionists" and believed that full restoration to eighteenth century conditions was possible. J.S. worked out a plan for Louisbourg that he submitted to the HSMBC in 1930. It called for a permanent, fireproof museum to house artifacts and research materials available to scholars, the reconstruction of important buildings, and the marking of streets. Webster wanted France and England to erect monuments on the site. However, J.S. contended that this would foster animosity between the two and believed everything on the site should be of the eighteenth century historical era or a replica of it.

The HSMBC, with the backing of the Department of the Interior, slowly began the work of excavation and preservation of the site. J.S., Katharine, Archdeacon Draper, and Mayor M.S. Huntington of Louisbourg were chosen as a special advisory panel on the Louisbourg project. Another person who was actively interested in Louisbourg was Albert Almon of Glace Bay. He would contribute his knowledge of the area and contacts in other communities in trying to retrieve displaced Louisbourg artifacts. These artifacts, plans, and documents, collected over the years by the McLennans, Webster, and others, would finally have a home when the Louisbourg museum was finished.

It had long been a goal of the McLennans to have a museum to house and display authentic eighteenth century relics from Louisbourg. Finally, in 1936, the museum was finished. The opening ceremony was held in 1937 with Governor-General Lord Tweedsmuir presiding. The building itself was masonry with a green copper roof. The building is still used as a museum today and contains the scale model of the town of Louisbourg that Katharine worked on for many years. The time and effort that Katharine put into the museum were extraordinary. Besides using her contacts and financial resources to locate and retrieve lost artifacts, Katharine was also involved in more tedious tasks like piecing together fragmented bits of china found at the site and sewing the French and English coats-of-arms into flags that hung over her model at the museum. Katharine would also write the site's brochure, "A Short History of Louisbourg", and when copies of J.S.'s book became hard to find, Katharine financed several new editions. She was named Honorary Curator of the museum and served in this capacity for twenty years. She was responsible for cataloguing artifacts, organizing the displays, and coordinating special events. In a later interview, Katharine would jokingly complain about the word "Honorary". She said that it sounded like she never did any work. To the contrary, the amount of work she did do and her influence over the Louisbourg project was attested to by John Lunn, Superintendent of Fortress Louisbourg, in a 1971 interview:

Singlehandedly, she [Katharine McLennan] founded and financed the Louisbourg museum, and was the active curator for twenty years. For many more years she was a member of the committee for restoration and in her ladylike way she persisted with the authorities until something was done.¹²

Katharine's commitment to preserving the history of the area continued long after the museum was erected. The process of retrieving Louisbourg artifacts continued and with the help especially of Dr. Webster, many more important relics were found. During excavation a 1720 French medal was found that had commemorated the founding of Louisbourg. Also, a lead plate was discovered that dated the building of the lighthouse at 1731. Scavengers over the years had uncovered and removed many artifacts from the site, including bombshells, cannon and musket balls, flints, locks, keys, and coins. Katharine began the long and undoubtedly expensive task of trying to track down these relics. The more impressive Louisbourg artifacts were tracked down and negotiations began between Katharine, the HSMBC, and the owners to bring them back to Louisbourg. These artifacts included the Louisbourg Cross at Harvard, the cannon at Halifax, and the Louisbourg bells, all booty taken by the New Englanders and British in the 1745 and 1758 sieges,

All of this work did not begin with being named the Honorary Curator. She had been researching and collecting artifacts and information alongside her father while he was writing his book, and later as she worked on her two models of the fortress. The Cape Breton Regional Library possesses the Louisbourg Collection that consists of the McLennans' original research. In looking through this material one is amazed at just how committed J.S. and Katharine were to their work. They hand copied documents found in the archives of Canada, France, and Britain. Included in their research papers are precise sketches of maps and plans of the fortress and beautiful copies of historical portraits of men like James Wolfe, William Pitt, Governor Shirley, and so on. The file also contains correspondence between J.S. and individuals such as Mary Rollins of the Boston Public Library, who helped with information and documentation on certain Louisbourg questions. The sheer amount of hand copying would have taken hundreds of hours. Some original materials were donated by persons who had inherited the personal papers of ancestors who had been in Louisbourg. One of the more outstanding examples of the McLennan's exhaustive and meticulous research was a large and very detailed chart which outlined the activities, movements, and weather conditions experienced by twelve British ships during the spring of 1745 at Louisbourg. The McLennans prepared bibliographies about Cape Breton and Louisbourg, acquired maps and indexes of the area, listed all the regiments and officers that had served at Louisbourg during the sieges, noted the names of ships, their officers, tonnage, number of guns and crew, and other information that would shed light on the Louisbourg of the eighteenth century. This research was done for J.S.'s book, but would later prove invaluable for the restoration of the site and the completion of the museum. Undoubtedly writing about Louisbourg made people aware of its importance, but in her "Short History of Louisbourg", Katharine explains her commitment to the museum: "The museum was built in the belief that a visual memorial is a necessary adjunct to a living past, and it is far more illuminating than the most eloquent words." An even greater "visual memorial" would be accomplished through the eventual restoration of the site.

Restoration would officially begin in the 1960's, but meanwhile, Katharine worked hard to preserve the site and mark historical events with commemorative ceremonies. In August of 1931, the Lieutenant-Governor, Malachy Daly of Nova Scotia officiated at a ceremony commemorating Brigadier-General James Wolfe's landing at Kennington Cove on June 8, 1758. A memorial cairn marked the site and about 1200 people attended the event. Among the honoured guests was J.S. McLennan, who was personally thanked for his commitment to the Louisbourg project. In 1937, two Brothers of the Order of the Hospitallers of St. John of God laid a tablet at Louisbourg in remembrance of the Brothers of their Order who died in the service of the hospital there. In 1940, the federal government raised the status of the site to the Fortress Louisbourg National Historic Park. Another event took place in 1958, two hundred years after Wolfe's landing, when the I.O.D.E. Louisbourg Chapter unveiled a memorial plaque to honour the event.

During all this time, Katharine pressed the government to allocate money for the restoration of the site. In 1961, a ceremony was held in honour of the twenty million dollar federal commitment. She admitted in an interview for a newspaper article in 1971 that she had actually been stunned to hear that the government was finally prepared to fund the work. Katharine continued her involvement in Louisbourg during the restoration phase. While she no longer held any official position at Louisbourg, her expertise and knowledge of the site made her an invaluable contributor to the process. She would often visit the site and investigate the work being done. One man who worked at the site, Herb Cryar, recalls that Katharine would arrive at Louisbourg wearing overalls and a brakeman's cap, often with Helen Kendall in tow. She would chat with the men and lend her expertise and opinions about the history of the site. Another friend, Mary Fraser, recalls that Katharine seemed to have an intuition about Louisbourg. She sensed that what the French plans had been for the fortress might not be what they had actually accomplished. Fraser contends that to hear Katharine speak of Louisbourg, one would swear that she had actually lived there in the eighteenth century. She was seemingly more familiar with the personages of that era than with many people of her own time. Katharine became friends with many of the men who worked on the restoration and, according to Miss Fraser, she made it her business to know all of them. She often entertained the various experts working on the project at her home on Whitney Avenue in Sydney and could converse with the men about Louisbourg as one expert talking to another.