Strict rules give me more freedom

Some years ago I was in Rotterdam on a kind of creative market – I forgot the name unfortunately – and there was a man who was selling his pictures consisting of only black line. They were either straight or 45 degrees tilted and by these very limited means he was able to depict faces of various animals, dogs, bears, horses, mice etc. 

I sat down at home and started to work out in sketches the characteristics of my cats look and finally I displayed it the same way this guy did. 

Strict simple rules are the best condition to be creative for me when I get lost in the tons of possibilities.