Great, excelent, awesome.
The criativity to create each enemy, each description and each specific level for each enemy is amazing. I died in the level4 in the first and second try, but I realized how to avoid the enemy, so in the next try I did (and I upgraded speed first for this level). Every single level is challenging, nice mechanics (I like the 'safe place' and 'Danger' idea) and nice upgrades.
I only think the cost of the upgrades is low, in the level6-7 you are full maxed.
Simple, but amazing game. Good job!