Sentinel 3: Homeworld is a sci-fi tower defense game. It is available for the Mac, iPhone and iPad, and it is tons of fun. Like in any tower defense game, you have to strategically place defense towers to prevent waves of enemies from attacking your base or reaching a certain point on the map. In this case, you are an invading force, and you have to hold down the enemies to protect your outpost.
There are many elements that make this game truly great. There are three game modes. I played the campaign mode. After you complete a level, you will unlock new weapons and abilities, which you aren't force to use. However, they will make it easier for you to pass levels. In each level, a sentry is placed on the map. This is an extra help, and you can assign it abilities. Some of them include auto-healing of barriers, and the ability to snipe down enemies. You place towers by dragging them onto the place you want to build them. They can be sold or upgraded.
The game can be played at normal speed, paused or even fast-forwarded. On the top-right corner of the screen, you will see the current wave and how many are left.
I love this game because you can play it for a long time, and there will be always something new that you can use. And the game doesn't force anything on you.
The Mac version runs very well and I found it very fun to play.