A classic star fort, the Unesco-nominated Benteng Belgica was built on the hill above Benteng Nassau in 1611, when it became apparent the lower bastion was an inadequate defence. The five massive sharp-pointed bastions were expensively crafted to deflect the cannon fire of a potential English naval bombardment. The fort is open sporadically, but guesthouses can track down the key-keeper.
It caused quite a scandal in Holland when, in 1796, the Brits managed to seize the fort (albeit briefly) without firing a shot. To reach the upper ramparts (with great views), take the second arch on the left from the central courtyard. Be sure to look for the old jail, where locals were imprisoned if they dared sell their spices to the English.