There are some photos posted over at halo.bungie.org of Fort Jesus located in Mombassa, Kenya.
Notice the eerie similarity of these pictures with Halo 2's Zanzibar map. No surprise that Bungie did their homework of the real Mombassa and modeled Halo 2's environments after real structures and locations. It's pretty cool to see the comparisons (if you can recall what the Zanzibar map looks like or have a screenshot handy):
