Fort Sumter was a federal property built to defend a main waterway of the continent from foreign invasion. If South Carolina had taken their secession to the Congress do sue for terms maybe they could have worked something out to legally secede and to eventually gain control of Fort Sumter.
Did we consider the forts that the Brits built in America to be their property after we had declared independance? I don’t think so.
Who built them, how much they cost, those are just details. You can’t have a sovereign state with foreign troops stationed on your soil without your permission. That’s called being in a state of war, whether the Northerners want to admit it or not.