See my #182. If someone was fortifying a fighting position in your living room, you would consider that an aggressive act. The North desired war, the South desired to be left alone, i.e. War of Northern Aggression.
Analogies are always so troublesome because they are always so inexact. If my motorcycle was in your hotel room and you decided to take ownership of the room - including my motorcycle - do I have an obligation to surrender it? The answer would be no, unless you present a clear and present danger to my safety.
Sumter was built on ground ceded IN PERPETUITY to the federal government. The structure itself was paid for by federal monies. It did not belong to South Carolina. Even though it stood within the boundaries of the state it wasn’t state property before the insurrection and it wasn’t cornfederate property after the insurrection. Because it was seized as an act of war it was considered booty until the federal government reclaimed it.
Perhaps we should rightly call it the War of Southron Aggression.