At the end of the Civil War troops that had garrisoned Fort Taylor were mustered out or ordered to other posts. Army engineers, though, remained and continued construction on the coverface. However, the project never received adequate funding and was significantly damaged in an 1875 hurricane, leading to a complete work stoppage. An Ordnance detachment also remained in the fort to maintain the cannon.

In March 1871, the Board of Engineers for Fortifications submitted a report discussing modifications to the fort. The Board recommended adding "two barbette 12-inch rifled guns, one in each of the sea bastions, removing ten of the present guns and modifying the parapet for this purpose." The report also said the main magazines should be strengthened against hostile fire. The report further stated that "the upper story of the barracks be removed, and the stories below, projecting beyond the terreplein, be covered by a flat roof of iron." The Secretary of War approved the plan in May 1871. A report to the Board of Engineers in December 1893 explained what progress had been made. "Two barbette platforms on the main work, with three sand traverses, one on each of the sea fronts, have been constructed, eighteen gun platforms were removed and the four magazines had been strengthened," the report said. These batteries held six 15-inch guns, two in the north battery and four in the south battery. As for the overall condition of the fort, the report said repointing and stuccoing were necessary, the wooden gun platforms had decayed beyond use, and the temporary wood shutters covering the Totten embrasures failed to keep out the sea water during storms. Many of the hinges for the original iron shutters were broken and rust had claimed the plates around the exterior openings of the embrasures. The brick quarters, the report said, were "in a deplorable condition." The report said a new roof was needed, as well as new doors, windows, ceilings, and "a general renewal of the floors, especially those of the upper story." There were also leaks and portions of the building were separating. The wooden part of the bridge leading from the fort to the cover face was also decaying. The earthen batteries to the north and south of the main works, constructed after the Civil War but never completed, had deteriorated beyond repair.

As the 1800s drew to a close, the nation again looked at harbor defense. Newer naval guns had been produced that, when placed on battleships and cruisers, had no difficulty in out-gunning the obsolete cannon of the Civil War. In 1885, President Grover Cleveland's secretary of war, William Endicott, chaired a panel that revisited the nation's harbor defense system. Known by the name of the Endicott Board, this group recommended taking a majority of Civil War era fortresses that had previously defended harbors and modernizing them with concrete batteries to allow newer weapons to be mounted. These newer guns had the range to battle hostile naval forces before those forces could reach the harbors.

While Congress was enthusiastic about these new defense methods, the enthusiasm waned when it came time for appropriations. The first monies for these projects didn't become available for Fort Taylor until 1896 when the likelihood of war with Spain loomed. And, with Spain having control over Cuba, the fort once again had a mission. The New York Times reported in an August 14, 1897 story that "Fort Taylor, which commands both entrances to the harbor, is being rehabilitated throughout, and will be able to offer some resistance to a hostile fleet of ironclads." The newspaper went to report that a subterranean mine will be excavated from the fort to the city that will be used to transport supplies to the troops or for the troops to evacuate from the fort. This mine would be one-half mile long with three openings; one at the fort, one at the quarter mile mark and one at the half mile mark. Also, according to Col. W.R. Kerr, disappearing guns would be placed on the recently improved sand batteries on either side of Fort Taylor.

Army engineers and civilian contractors began the process of converting the Civil War fort and surrounding military reservation to a more modern coastal artillery installation in early 1897. The first project would be construction of a mining casemate on the coverface. This was a fire control building for the mines that would be submerged in Key West Harbor. It was completed in late 1897 and remained in service until 1904 when it was converted to a magazine for gun cotton for the submarine mines. Also in 1897 civilian contractors would begin construction on batteries outside of the fort but located on the "Fort Taylor Reservation" on Key West. These batteries were built on the site of the old water battery at Whitehead Point, which is the geographic southernmost point in the continental U.S. They were completed, for the most part, by 1899 and emplaced with two 10-inch rifles on disappearing carriages and two 8-inch rifles mounted on disappearing carriages. These batteries were named DeLeon (for Ponce DeLeon, Spanish explorer and European discoverer of Florida) and Covington (for War of 1812 Brigadier General Leonard Covington). Construction of the mortar battery (Seminole) also began at this time.

In 1898 there was quite the activity at Battery Seminole. The New York Times reported in a dispatch dated May 28, 1898 that the artillery sergeant of the guard was shot at by three men. The newspaper said the sergeant discovered three men trying to enter the military reservation late at night. Upon the discovery the men opened fire on the sergeant with pistols. Three shots missed the soldier, but a fourth hit him in fleshy part of his left arm just above the elbow. The sergeant returned fire but the men had run out of range of his rifle. A search of the area turned up no men. Sometime after midnight the soldiers did find a carriage parked just outside the gate near the mortar battery. The men, according to the Times report, were said to be Spanish spies.

In 1893, soldiers other than engineers returned to the Key West Barracks. When war with Spain broke out in 1898, those soldiers occupied Fort Taylor. The unit that returned to the old fortress was the first to occupy it in 1861 -- Battery B, First U.S. Artillery. The soldiers remounted some of the old 15-inch Rodmans and large Parrott rifles. The declaration of war served as the motivation to complete the harbor defenses as quickly as possible. Construction on another Endicott battery was begun. Battery Gardner (named for Captain George Washington Gardner who was killed in action in 1835 that precipitated the Second Seminole War) was completed in 1899.

Work on the new Endicott Battery inside the fort began in 1898. These batteries would be built on the south and west face. The engineers first removed the top two tiers of the entire fort. Also, a section of the main barracks building was removed to create a new entrance into the fort. Before the newer batteries were built, the engineers took the remaining Civil War cannon and moved them into the south and west casemates. They then filled the casemates with sand, gravel and concrete. The idea would be to make the casemates strong enough to sustain bombardment and protect the new magazines. Two batteries were constructed--Battery Osceola and Battery Adair. Osceola is named for the Seminole chief against whom Colonel Zachary Taylor battled in 1836. Battery Adair was named for Lt. Lewis Adair, an infantry officer who lost his life fighting the Sioux in South Dakota in 1872. Battery Adair was constructed on top of and just behind the original casemates of the west curtain. Battery Osceola was built directly in back of the casemates of the south curtain. The batteries were completed by 1902.

Battery Adair was fitted with rapid fire three-inch guns on pedestal mounts. It was designed to protect Key West Harbor. But, by 1921, Battery Adair was listed on War Department maps as being abandoned. Seacoast mortars, each with a crew of a dozen men, were mounted in another battery that faced the harbor entrance. The picture on the left is a seacoast mortar battery at Battery Seminole, the remains of which are located on U.S. Navy property adjacent to Fort Taylor. Battery Seminole, Battery Osceola and Battery Adair are the only surviving structures of that era of coastal artillery in Key West.

Battery Osceola, because it faced south, was reserved for the heavier coastal artillery guns. Two 12-inch rifles were mounted here. Each piece required a crew of 43 men. It could fire a 1,000 pound round a distance of 10 miles and could be made ready to fire again within 30 seconds time. The munitions and powder for these guns was stored below. To bring up the rounds and powder, engineers had constructed a rail system along the ceiling of Battery Osceola. The rounds and powder were placed in trolleys that moved along the rail until reaching an elevator shaft. The rounds and powder were then transferred to an ammo lift in the shaft, raised up, and then transferred to another cart on the second floor where they would be moved to one of the two guns. The bottom floor of Osceola housed the magazine and storage rooms as well as the engine rooms to power the installation.

Other batteries that were built on the "Fort Taylor Reservation" included Dilworth - armed with two 3-inch rapid fire guns (named for 2LT Rankin Dilworth who died of wounds received in the Mexican War of 1846), De Kalb - armed with two 6-inch guns (named for Major General John Baron De Kalb of Bavaria who served with the Continental Army during the American Revolution) and Ford - armed with two 3-inch rapid fire guns (named for Major Mahlon Ford who served in the Revolutionary War as an artillerist and Engineer). An additional battery (Inman) was built at the West Martello Tower and remained in service through World War II.

The coastal artillery weaponry, manned by various units of the U.S. Army's Coastal Artillery Corps, remained at Fort Taylor through mid-World War II. The Army then built new mounts on Battery Adair and the old north curtain of the Civil War casemates for 90mm anti-aircraft guns. Those were removed in 1947 when the Navy took over the property from the Army.

Source: Gaines, Military Engineering on the Florida Reef: The Coastal and Harbor Defenses of Key West, Florida 1829-1946