Take a basic shoot’em up, take away any form of stages and only leave the bosses. Then add an element of adaption. Meaning, how you fight and kill a boss will determine how the next boss will evolve. The first boss is a little thing, easily defeated, but soon the bosses will grow to enormous size and deploy an extensive weapon arsenal. So, you can’t just defeat the bosses, you have to think how to defeat them, if you want to have any chance of beating the next one. Because if you fight every time the same way, the bosses will adapt to your tactic, and if you don’t change and adapt yourself, it’s over pretty soon. Very addictive and fun to play.