Bulkypix has released a universal iOS version of its classic tower defense game Grave Defense HD. Originally made popular on the Android platform, Grave Defense HD sets the player in a post-apocalyptic world where they must defend their base against hordes of the undead along with some of history’s most notorious monsters. Following a traditional tower defense model, players must build weapons towers and upgrade them to fight off the zombie invasion across a number of unique locales ranging from the Amazon rainforest to the Bermuda Triangle and outer space. As the players advances, additional towers and upgrades become available along with new types of zombies and monsters to contend with. The iOS version also adds a number of unique new features, including a survival play mode with more tower upgrades, an online ranking system, visible monster paths, tower meters and an iPad-specific heads-up display view. Four difficulty levels provide challenges for both casual and hardcore players. Grade Defense HD is a universal app and is available from the App Store for $3.