When we move Tower action logic into the tower (and out of the LevelPanel), we will need the Tower to have awareness of the Level so it can do stuff like findAllEnemiesInArea to shoot at.
Simple solution, Tower (and probably eventually Enemy) have a setLevel on them?
On a related note, the name LevelPanel would be better as "Level" in my book (if we were separating view/model then we'd probably have both). I'm easy on the last though.