When you complete a level you have the option of continuing forward.

when you go on to the next level you can go back to the level by walking back through the door you came through.

When you do all the monsters you defeated before are back, but don't worry because you don't have to beat the level again to go forward.

The door is still open so you can walk trough at any time.

This allows you to gain as many souls as you need on early levels.

I advise that you don't try this on later levels due to the fact that it is really hard.