New York City was not large enough to provide a safe location to store thirty million surplus munitions cartridges, in the spring of 1901 he began to build an arsenal on Pollepel. Bannerman designed the buildings himself and let the constructors interpret the designs on their own. Most of the building was devoted to the stores of army surplus but Bannerman built another castle in a smaller scale on top of the island near the main structure as a residence, often using items from his surplus collection for decorative touches. The castle, clearly visible from the shore of the river, served as a giant advertisement for his business. On the side of the castle facing the western bank of the Hudson, Bannerman cast the legend "Bannerman's Island Arsenal" into the wall. Circa 1900–1910 the US Navy re-gunned a number of older ships. Bannerman bought the old guns, presumably at scrap value. In 1917, following the American entry into World War I, Bannerman sold a number of these weapons to the US Army, which intended to mount them on new carriages as field guns for the Western Front. Thirty 6-inch (152 mm) 30-caliber ex-Navy guns were acquired by the Army, but sources do not indicate whether any of these were sent to France. The Army's 6-inch gun units in France primarily used former Army coast defense weapons; none of these units completed training and thus did not see action. Construction ceased at Bannerman's death in 1918. In August 1920, 200 pounds of shells and powder exploded in an ancillary structure, destroying a portion of the complex. Bannerman's sales of military weapons to civilians declined during the early 20th century as a result of state and federal legislation. After the sinking of the ferryboat Pollepel, which had served the island, in a storm in 1950, the Arsenal and island were essentially left vacant. The island and buildings were bought by New York State in 1967, which took possession after the old military merchandise had been removed, and the relics given to the Smithsonian. For a short time, tours of the island were given in 1968. However, on August 8, 1969, fire devastated the Arsenal, and the roofs and floors were destroyed. Following the fire, the island was placed off-limits to the public. Current status In the 21st century, the castle is the property of the New York State Office of Parks, Recreation