There are doors that require many keys to open. Is it possible to get keys as drops or do you have to purchase them?
From the official Support page (click on "Stars, Keys and the Gates they Open!"):
If you don't have enough Keys to unlock a gate, you can replay levels to earn more. Keys may appear in any level or mini-game, as they are dropped at random by zombies. Keys also may be among the treasures found in Yeti Zombie Lunchboxes.