Zombie mode shipped with the initial release of the game, but the creators decided to keep it hidden for a while.
Buy it on Steam: http://store.steampowered.com/app/45450/
What it unlocks?
You get a totally new campaign, same maps but this time with zombies chasing after you. Zombies constantly spawn from the grave which in normal mode represents the coat of arms of the castle. Zombies can kill you the touch you even on the baseline so you have to be extra careful.
The Zombie mode is easily accessible via mousing over the bird resting on the sword at the title scree, many times in a row.
Afterwords doing this once, it is possible to take the lazy route and use the new button on the bottom right of the screen to switch between Classic and Zombie mode.
Below you will find a video demonstration.