Well, turns out I've apparently explored every single room except what I assume is the final boss. Why do I always play games like this

If you feed any dude to a red bug then the red bug goes away. Just jump into the bug's mouth.

Is there any way to kill the red bugs that span multiple screens??

For those confused about the bottom right (and the key up in the top right) many of the brown square block tiles can be blown up with Bomber.

The game doesn't really hint at this too overtly, but you can see in some of the tiling patterns in Castle Nilpis that there are sections of square bricks that seem out of place. If it deviates from the usual one-tile border around the typical room structures there's a chance you can bomb them away.

Due to this the bottom right area is a fairly easy way to "cheese" getting into the Dark Heart room. As long as you get all the switches, you can make your way up and bomb into the chamber.

How would you get the key in the top right? I feel like I missed a mechanic cause the bottom right area seems completely pointless and that key seems inaccessible

As soon as I saw the bottom right and noticed what the switches do this game became an easy cherry.

Holy shit!
That map WAY bigger than I thought!