Scroll down on the store page and expand the Pre-Purchase section to reveal bonus items!
http://store.steampowered.com/app/262280
Pre-purchase before April 24 and receive:
- Digital Soundtrack & Digital Art Book for free!
- Dungeons 2 - Morningwood DLC (Exclusive to Prepurchase) for free, to include a new single-player skirmish map!
- Dungeons - Gold Edition & Rise of Venice for free!