Download Drop Defender
Drop Defender is a game that is built as a tower defense real time strategy one. There are a lot of levels that you need to pass in order to completely beat the game, and it will take you some time in order to progress. As with any other usual game, there is a regular increasing in the difficulty of the levels, meaning that you will struggle towards the end.
There are 8 different maps on which you can play and in order to protect your base you have access to 6 different turrets which also have 9 possible upgrades each. There is also a built-in browser for the game, allowing you to see the scores of others.