This short stage is unusual because it contains no enemies whatsoever, aside from the boss.
You have to break the fragile blocks to make the tower collapse, so that you can get to the boss's lair. This is tricky due to the spiked ceilings, so use caution when you shoot the blocks. You don't want to get speared from above! It just takes a little patience and thought, but you'll reach the boss room before long. Just be warned, if you lose to the boss, you have to repeat this section.
After a brief cut scene you'll find yourself face-to-face with the fourth and final Dark Man. This red guy has several skills of his predecessor, and then some. He uses a shield similar to the second boss of the fortress, and is even deadlier because he can fire his shield out at you, which will then boomerang back to him. One strategy is to use Beat to hunt him down, while you fire with your Mega Buster. He's technically weak against Star Crash, but it's very hard to hit with, so I don't recommend it. Just stay on your toes, and you should best this guy.
Onward to the second fortress!