Before entering this stage, remember you need to be cautios about your keys. Try to keep some in reserve, and only open the lockers to refill one weapon. Also, avoid any deaths, as there's a chance you'll lose one of your keys, even inside the enclosed boss arena.
In this stage, you have a choice of four ladders. On all four ladders, enemies will approach from above and below. If necessary, you can jump from ladder to ladder to avoid some difficult enemy combinations. Regardless of your choice or movement, you should have at least six keys.
At the top is the boss room. The boss is a simple turret with a centipede-like tail, and it will shoot a single red shot at you every few seconds. After a few attacks, it will park at the lower part of the ladder. Get ready to jump over it when it starts to move to the side.
As the enemy takes damage, its tail segments will be destroyed. When it is destroyed, shoot the skull, and climb through the opening.
In the intermission, you can unlock members of your family for 1 key each. Two of them drop weapons, the 3-way and the shotgun. However, family members that you don't rescue won't appear in the ending.