Sentosa island was previously named Siloso Island, after a tall hill on the western edge named Mount Siloso. In the early days of British involvement in Sinapore, the top of Mount Siloso was literally blown off to make way for a strategic fort to defend the Sinapore harbor. Over the years the fort grew and matured, and eventually served as the British Empire's strategic naval defense of Singapore in World War II, as the most likely attack from the Japanese would come from the sea. Of course, the Japanese are tricky, and instead came by land, rendering the fort's powerful naval cannons essentially impotent against the advancing forces (they're like rifles, whereas they needed shotguns). Combined with an undermanned force, some strategic blunders, and just overwhelming opponents, Singapore quickly fell. The British surrendered to the Japanese and a massive POW camp was set up on the island's eastern edge, many prisoners of which died in the construction of the "Highway of Death" through Malaysia, Thailand, and up to Burma.
Regardless, the defenses at Fort Siloso are remarkably well preserved, especially its underground bunkers and ammunition depots. I spent most of my time wandering alone through Sentosa (other than the ticket lady I didn't see anybody working there, and only a few other visitors occasionally scurried past) and really enjoyed the exhibit. Throughout the entire walking tour there were signs advertising and hyping up the audio/video production of the invasion of Singapore in WWII, and I was getting excited for the tour's grand finale. Every exhibit slowly built up to this massive crescendo, and the last exhibit's (the westernmost cannon, as shown) dramatic audio reenactment made my heart race as the edge of fear creeps into the soldiers voices as they start to realize impending doom - only to find that the movie was down for repairs. Bah, what an anticlimax.