I don’t think you can disassemble walls or floors. Your best bet is probably C4, or something even stronger.
As for fortresses, your best bet in town is a fire station. They have brick walls (stronger than most), metal doors (VERY strong) and a garage space with a garage door. Get one with intact winches and get some lights, maybe some defenses, installed and you’ll be pretty damn well off.
You could also consider a military bunker or science lab. Bunkers are nigh-impenetrable but don’t have much space on the surface (plenty underground). Labs are a bit bigger on the surface and if you’re lucky and get an icey one you’ll be comfortable through summer and can store stuff underground to keep it permanently frozen. They’re not quite as tough as bunkers but are still going to handle pretty much anything that can get thrown at them.
That said, neither have garage space so until you have some defenses on your vehicle you may want to hold off on them.