Fort Scratchley in Newcastle, New South Wales, Australia is a victorian era coastal battery with some nice underground galleries. It's not too big to defend, PCs could manage it.
Go to the link above, there is ground plans of the fort. If you don't want to use the fort as a coastal battery, just site it on a bluff looking towards the probable direction of attack. It's international design makes it perfect for anywhere in the world and it is small enough to to be overlooked on many maps.