FG built his fortress covered in cannons to ward off attacks. And uses them on any who attack.

The First Fortress boss is the Fortress Door, yes, he lets them enter through the front door.

During the stage you must evade cannon fire. At some point Mimicman attacks you, and the cannons stop for the fight. If you are playing Dr. Fox, Folxbot, Prof. Chauve or FG Mimic will be Millie, if you are Millie he will be Folxbot.

The Boss attacks with the three cannons over head, to open the door and defeat the boss you must shoot and destroy the red power orb on the lower area of the door. Weakness to "Destruction Laser"