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There is a lot of history here, including an outline of the original wooden structure (circa 1797), earthwork forts from the Civil War era (finished 1861), the large granite fort (construction stalled out in 1863 but for the most part finished later), military museum and tank, and so on.
The main hallway through Battery Milliken and the hills themselves are all traversable. The small artillery hill mounts and bunkers, which have rooms to explore, are safe and the city cleans them up every so often. Then of course there is the largest granite fort, Fort Taber, which looks like a proper fortress, closed to the public except for events, such as pirate attack or Revolutionary reenactments.
Know Before You Go
This is an unsupervised site. Explore at your own risk. Everything is fine except when fools try to go down the staircase to the lowers levels of the Battery Milliken (largest hill fort with the graffiti), there's water down there... Plus, down the side halls, it is as dark as anything can be, so it is almost impossible to navigate anyway.
It's a fantastic place to stroll and explore. It has a fishing pier, beaches (with lifeguards and parking - East Beach), BBQ areas, etc. Features reenactments at times and public outside concerts next to the sea.