Inside the tower there are two locked doors and a green cocoon.

Shoot the cocoon to wake the sleeping monster.  For each monster you wake, another cocoon appears.  Wake three monsters and the doors will unlock.

The red monsters will shoot at you if you stand directly in front of them (on the highlighted tiles).  You can’t do anything to hurt them, so don’t bother shooting back – unless you enjoy making dangerous monsters angry.

When the doors are unlocked, climb the stairs and go through the upper exit.