To pass this level, you should pop the pig that in the rocky tower. But this level is not hard, you have one ice bird to help you destroy the fortress.
Total: 3 birds and 1 pig.
Send the Ice Bird straight ahead, into the bottom part of the structure. On contact, he freezes everything he touches. Also, after 2 seconds, or upon manually triggering, a cold blast freezes everything in a small radius. So, when you send him straight ahead, trigger his ability upon contact. Then take out the structure with the Blue Birds.