One of the most amazing artifacts on the Galveston island is what remains of Fort San Jacinto.
The construction of the fort at the eastern tip of the island began in 1897 and was garrisonned on April 28, 1898 by Battery G, 1st U.S. Artillery. There were four batteries built. The Great Storm of 1900 badly damaged the fort and it was abandoned for a few years, until 1911 when it was repaired and re-garrissonned.
The circular structure at the edge of the parking lot used to be the mount of 90-mm Gun No. 4 of Battery AMTB. The battery was built during World War II in 1943. It was decommissioned just a few years later, in 1946. This is the only mount that is visible today. Three others were where the parking lot is now.
Be careful making your way to the cache. Beware of all usual critters (especially snakes), but also the huge rocks under your feet. The cache is very close to a trail, but these few feet can be tough.
Sources:
Fort Wiki - Fort San Jacinto
Texas State Historical Association
The rise and fall of Fort San Jacinto (The Daily News)