Here's the official description from Rovio:
It is a dark time for the Rebellion. Evading the dreaded Imperial Starfleet, a group of freedom fighters has established a new secret base on the remote ice world of Hoth. Unfortunately the evil Darth Vader discovers their hideout, and the desperate Rebel birds must escape the AT-ATs and Pigtroopers hot on their trail.
The free update brings 20 new levels, two bonus levels and introduces the pink bird as Princess Leia. Like the other characters we've been introduced to, she brings a unique ability to the battlefield – a tractor beam that can be used pull the legs right out from under the dreaded AT-AT walkers.
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