I am? Was the property for the Fort was given to the federal government - or was it purchased by the federal government? (Hint: it was NOT purchased by the federal government).
From Article I, section 8, clause 17: [T]o exercise like Authority over all Places purchased by the Consent of the Legislature of the State in which the Same shall be, for the Erection of Forts, Magazines, Arsenals, dock-Yards, and other needful Buildings."
In Federalist 43, Madison explains that it's "public money expended on such places". Public money (eg, federal monies) did not purchase the land in consideration. It also had to be done with the consent of the state legislature, and we all know that legislative acts are subject to rescission (no prohibition against that either).
Since it was an artificial island, it REALLY belonged to the Federal Government--they built the damn thing in the first place. And other facilities that the Confederates had chased the Federal government out of had been secured in fee simple.
So, which one is it?