Somewhere, I seem to remember reading something that pretty much said the forts would be maintained by the Federal Govt. for the protection of the United States even if they were built or the land owned by a specific state. The land and fort itself was the property of the state if the US had not paid for it. This was sometime after the Revolutionary War and way before the Recent Unpleasantness. I do not though, remember where this was located. For some reason I thought it may have been on this site. I believe the issue was brought up not about coastal forts but those in the interior that later were considered not viable. I think the wording though, did include ALL forts, not just the inner region ones. I don't think at that time they thought there would be a state or states that wished to leave the 'protection' of the US Govt. South Caroline always maintained they owned the forts. After Sumter though, it looks like physical posession was the deciding factor.
I have information on Ft Pickens though. The US purchased it from my 3x Great-Uncle and hubby's 3x greatGrandfather for the sum of $10,000. Wish we had a family beach shack there now. Darn it. :(