Kinda simple. Use only mouse to move/fire. There are two options in the main menu. Mission mode and Survival mode. In mission mode you "invade" in some territories where there are some enemies and a boss in each mission. I was surprised how good enemies combos appear. In last missions, combo of enemy units become very hard to beat. Also appear time to time a friendly unit to send some upgrades such weapon damage, fire rate, shield etc. Notice that you have only 2 life's to finish each mission every time so be careful. Final bosses are kinda easy but most times you need more than one life to beat them so be a lot careful in each mission. Shields also helps a lot. Also spend nuclear bombs when you have them. Don't keep them cause you can't use in final bosses. After each mission you can upgrade up to 3 levels some extra weapons of the tank. Spent points wise here cause it helps a lot in each mission. The other mode is survival mode. Here you start in an endless game and the point is to survive as much possible. And in this mode the friendly helicopter assist you with the same upgrades as the mission mode but it also drops special upgrades same as the one you pick after each mission.
Graphics are 2d but are very well designed and the effects also pretty good. Don't wait for something amazing but the designers made a very nice job here. The sound also fits well with the game.
In general lines I found the game very simple and addicted. Combine with the hard enemies make this game perfect to spend some time when you want something short and challenging.