Of course it's never that easy. In order to do so you'll first have to locate the exit on each level and then plan your exit or you'll just get butchered.
This mean that you'll build turrets and other defenses in rooms and you'll also find the best equipment you can in order to be as ready as possible.
Once the crystal is taken, all hell breaks loose and ennemies spawn from everywhere trying to overwhelm you. And boy, it's tough.
The game might let you choose an "easy" difficulty at the start but I'd recommend the "Very Easy' one or you will smash your iPad against the wall.