Fort Rodman and Fort Taber are the same place, so not sure what you mean by that?
Yes, I was going to say the same thing.
You can get into Batteries Milliken and Walcott... Milliken is the largest battery, the easiest to get into without being noticed, and by far the most interest. Walcott is small, a little boring (though worth a look if you haven't seen it before, I supposed), and nestled in the busiest part of the park. That being said, you won't likely be stopped from going in as you would Milliken, and they've made no attempts to close it off. I've heard they rarely open Fort Rodman and allow visitors in to see it, but in all the times I've been there I've never seen it open. My brother swears he has.
The other batteries are all filled in, I believe. You used to be able to explore a side room of Battery Cross, but they cinderblocked and mortared the doorway 4 or 5 or so years ago. They had done the same thing to all the doors of most of the other batteries, but someone had tunnelled in through the top of of Battery Barton sometime between 2004-2005. I went in with my little brother to check it out and a couple of friends waited outside.
We climbed down... the entrance opening was pretty narrow, but it widened considerably into a grade that brought you down to the ground level. It wasn't very large, but there were at least two rooms and the hallway made a U-turn that I didn't get the chance to explore. About half way in, I noticed that the wall was covered in house centipedes (
Scutigera coleoptrata). I mean, the walls looked like they were fucking ALIVE because of them. These happen to be the only animals that I'm irrationally afraid of (I think we all have one), so I really started privately freaking out and sort of ushered my brother out of the battery without explaining why. I noticed on the way out that the centipedes were also lining the very narrow tunnel way and hesitated coming out for a few minutes... when I got out, I explained why we left and he didn't seem to mind. It was very odd. I've never seen centipedes congregate like that... they aren't social animals at all. I can honestly say that I saw more down in that battery than I've seen the entire rest of my life, and I've seen a lot.