Very interesting read - I had not known that the British Empire maintained forts on US territory after the Revolutionary War, with their status not finalized unitl 1814. I’ll have to read more about this.
I do agree with the contention that Guantanamo is similar to Sumter, though. In both we have a legal right to be there, and an unprovoked attack on either would be considered an act of War.
Somewhere I saw a detailed report and map of these forts, but a quick google only gives me this one: British forts on United States land, 1783 to 1814.
And here: