Of course, firing on a federal fort is not an illegal act against the Constitution. You may want to refresh your memory. Read Article III section 3 of the Constitution of the United States.
Well suspending habeus corpus, arresting and jailing sitting congressmen and attempting to arrest and jail the Chief Justice of the Supreme Court seems a bit more worrisome than running the yanks out of your territory after you have announced secession. :-)
I’m trying to remember the Confederate invasion of New York and Boston. LOL!