I've been doing close to.. 50 runs now? I unlocked all the characters except the last one and farthest I got was this "castle"-thing... Any tips on what the best character is?

The fireball is very powerful, as its special attack is very cheap and means you don't have to worry about running out of bombs. But I usually do best with the vampire bat, which is really powerful if you're careful and make sure to buy a health upgrade or two as soon as you can. The slime is strong as well, killing most things in one hit is very useful. 

I think those are the strongest three, but it depends on how you play, what upgrade you like, and a little luck.

Yea, I've managed to defeat the king now with the fireball :)

But man - the queen... You have to have the perfect run. Does anybody know what you get for defeating her? I don't wanna grind for 3 hours to get to her...