To increase the difficulty of each level, I slowly introduced new features, such as enemies, vines, spikes, and combinations of blocks.  This gave players new hazards and tools to work with while never overwhelming them with the added complexity. 

Above, the player is about to push a large stone block on top of an enemy, squishing him. The player learns in this level how to use vines to climb as well as how to defeat enemies by using the stone blocks.