There are thousands of Android apps free for download, as well as games which you can play for hours and days. But if you like to play a mix of the traditional tower defense games and real-time strategy games, then you better try Dungeon Keeper, now available for Android. 

As a direct successor of the classic strategy game, Dungeon Keeper remains faithful to the original. Players can create their own dungeons or lairs to keep enemies away. And instead of towers, they are able to summon ancients, monsters and minions to do their bidding. 

The game offers a number of modes which include the classic 'tower defense' mode, real time attack against other players, and team up with colleagues to unleash the ultimate attack against other guilds. 

What's best about this game is it comes absolutely free for download from Google's Play Store. 

However, you may want to prepare your budget as some features in the game can only be unlocked by in-app purchases. 

Nevertheless, progressing in the game can still be fun and challenging even without spending anything. Indeed, Dungeon Keeper will bring out your worst side in order to crush your enemies and defend your lair.