Each level adds one more enemy - the first level just has one enemy, Yeerks, and the fourth and last level has four enemies - Yeerks, Taxxons, Hork-Bajir, and Blade Ships. Once you find the last key in Level 4, you've made it to the Yeerk Pool. You have one minute to unscramble the four keys to spell an animal's name (such as WOLF, DOVE, FISH, or TICK) to unlock the cell and free the Animorphs. When you successfully unscramble the word, you have won Hawk Rescue.