The new version notably brings an arena mode, which hearkens to gladiatorial combat. Competing with others is encouraged, as is creating zombie-killing structures. Also, the leaderboards allow for direct comparisons, and there are new zombies and environments. Additionally, starter packs are now half off.
We had an opportunity to look at the game a while back, and enjoyed it.
Dead Trigger 2 is free-to-play on the App Store.