Could you have a drawbridge?
Sure, a drawbridge is just basically a garage door with a motor sort of thing that winds up a chain. The hinge is on the building and matches the shape and size of the opening. It's definitely a good way to keep people out—that and alligators.
Oh right, could you have alligators?
I think you need a zoo permit for that. A friend of mine has a zoo down in Greenwich, Connecticut and it’s a pretty big deal. Might as well just do it and see what happens. You’d probably just need someone to tell you how to feed them. If the appeal is to have everyone eaten by alligators, that would be different.