The popular sci-fi tower defense game Sentinel 3 is on sale for only US$0.99 for a limited time. Normally $2.99, the sale price is in celebration of the release of a massive new downloadable campaign that is available as an in-app purchase for $1.99. The new 15-map campaign features 15 endless maps and 6 new classic maps, as well as a new commander unit, new weapons, new turrets and new enemies.
In addition to the new in-game features, Sentinel 3, which is a universal game for the iPhone, iPad and iPod touch has beefed up the iPad graphics with higher resolution background levels. The game's engine has also been completely revamped for much-improved gameplay. No word on how long Sentinel 3 will remain on sale, so if you're a fan of tower defense games, you might want to snap this one up quickly.