Fort Siloso has a fair concentration of guns ranging in ages from older Bottle guns through to the WWII vintage (plus captured Japanese examples on static display), plus the bunkers are mostly open with stuff inside them.
The core section where the tram drops you off houses a chunk of museum sections and the old monorail station houses the Surrender Chamber waxwork collection. Its one of my must see stops whenever I visit Singapore on one of the days I hit Sentosa and I'd seriously recommend it. Some of the details of the 'Stay-behind' force is quite startling
The influx of the laser tag game site can get in the way – one section of tunnels and access to the lower down lookout point and gun point wasn't accessible anymore as its part of the laser game area last time I was there. That's most of the section under the 'Siloso POint and 12 pounder' shadow in the fortsiloso fansite listed above.
If you want to explore then Sentosa still has a chunk of other WWII remnant. Fort Connaught is buried under the golf courses so you essentially cannot get at that – but the Mount Imbiah site was being cleared some and you could get some access to an unrestored battery site there. When you go down to the beaches there are a couple of pillboxes still visible, an MG nest is clearly visible and marked up by the beach road.
As for the rest of Singapore there's a whole parcel of WWII stuff – the National Museum has a small collection in the main gallery. You have the Johor Battery, Labrador Park (though the tunnels are still closed I think), Reflections at Bukit Chandu is a fairly powerful monument. You have 'Memories at the Old Ford Factory' where the British forces surrendered and of course if you have any interest in these things I have to recommend Changi Museum
.that's not the complete list of WWII related items either.
As an aside the BattleBox is closed until further notice so don't chase after that even if you get to Fort Canning
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