Can goblins pull levers?
Goblins cannot pull levers, so you don't have to worry about levers on the outside of your fort. The only creatures who will pull levers in DF fortress mode are are dwarves with the lever pulling labor enabled and gremlins, since gremlins have the MISCHIEVOUS tag.
If you want some extra !!FUN!! when playing in DF fortress mode you can mod the goblins raw files so they also have the MISCHIEVOUS tag, which will then allow them to (randomly) pull levers they find.