Battery Laidley is the heart of Fort De Soto and the reason the place is remembered. It is a great earth-covered mass of concrete holding eight 12-inch mortars, weapons designed to lob half-ton shells high into the sky to fall straight down through the thin deck armor of a warship. Four of the eight are still in their pits, the only carriage-mounted 12-inch seacoast mortars left in the continental United States, and they have never once been fired at an enemy.
A mortar is not a gun in the ordinary sense. Where a gun fires more or less flat, a mortar throws its shell up at a steep angle so that it comes down almost vertically. The logic was precise and deadly. A warship of the era carried its heavy armor on its sides, where it expected to take fire, and only thin plating on its decks. A shell dropping out of the sky at the end of a long arc would punch through that deck and into the ship's vitals. Battery Laidley's eight mortars, set in two open pits of four and meant to fire in coordinated salvos, were designed to bracket a target in the channel with a rain of plunging shells, each one weighing roughly half a ton, out to a range of about 6.8 miles.
The battery itself is a study in concealment and protection. The two mortar pits are flanked and separated by magazines, machinery spaces, and plotting rooms, arranged in a line and buried under a hill of sand and earth so that from the sea there was nothing to aim at, only a low green rise. The interior rooms, windowless and described as dungeon-like, held the shot, the powder, the firing and reloading gear. Behind each pit stood a small brick booth from which fire-control data was displayed to the gun crews. The walls run up to twenty feet thick.
Construction ran from November 1898 to the spring of 1900, the work of an industrial-scale plant erected on the bare island: a wharf, a narrow-gauge railway, an overhead cableway, a concrete mixing operation. It is here that the fort's most famous detail belongs. When the stone shipments from the north ran late, the engineers substituted crushed beach shell as aggregate in the foundation concrete, and it worked so well they used it throughout; the shells are still visible in the weathered walls today. The battery was reported complete on 10 May 1900, and the final accounting became local legend: of the roughly $155,000 allotted, just $16.73 was left unspent.
By War Department General Order No. 78 of 25 May 1903, the battery was named for Colonel Theodore T.S. Laidley of the Ordnance Department, a veteran of the Mexican War and the Civil War, an authority on ordnance and gunnery, who had died at Palatka, Florida, in 1886. The mortars were test-fired on 19 and 20 November 1903; the concussion is said to have rattled windows for miles. It was nearly the only time they ever spoke.
Coastal mortars were obsolete almost as soon as they were emplaced, overtaken by faster ships and longer-ranged naval guns. When the United States entered the First World War, four of Battery Laidley's eight mortars were dismounted and shipped away for the war effort, which is why four remain. Those four are now the only carriage-mounted 12-inch seacoast mortars left in the continental United States; the only comparable survivors anywhere are at Battery Way on Corregidor, in the Philippines, and those are battle-scarred ruins from the fighting of 1942. Two of the Fort De Soto mortars are displayed deliberately, one raised in the elevated firing position and one lowered in the depressed loading position, so a visitor can read the whole cycle of the weapon at a single glance. They are, in the end, the guns that never had to fight, preserved precisely because the war they were built for never came to this channel.