Kingdom Rush

First, you will have to comoplete the regular campaign. This will unlock several bonus levels.

On the top-right side of the map there is a level called Rotwick. Beating Rotwick unlocks Ancient Necropolis. Beating Ancient Necropolis unlocks Nightfang Swale. Beating Nightfang Swale unlocks CastleBlackburn.

Lord Blackburn is the final Boss on the level Castle Blackburn.

You can also get the Nessie achievement of Nightfang Swale by clicking the left thumbstick to change your cursor to a hand. Next, look for bubbles in the green water by the hut. Click on the bubbles to find Nessie.