Other than cheating, anything and everything is allowed.
The wilderness includes ANY land that isn't claimed by another player. If land was claimed at one point and then unclaimed, it will automatically become part of the wilderness.
There are no limitations on breaking blocks, placing blocks, opening doors, taking items from chests, and attacking players. There is no way to protect property in the wilderness.
Anything built in the wilderness can be destroyed by anybody else. Anybody can be attacked at any time, unless PVP combat is disabled by the server.