If tower defense games are your thing you might as well just go ahead and fire up BlackBerry App World on your BlackBerry PlayBook right now. GRave Defense HD has landed for the BlackBerry PlayBook and is now available for only $2.99. Another stellar game brought to the platform care of Marmalade. Take Field Runners -- add in some zombies with set paths instead of them being free-roaming and a ton more weapons and you've got GRave Defense HD. If you're looking to see the game in action, you can jump past the break for a look at the Android version's trailer as the gameplay is the identical.