Tactical Retreat is a Wave Based Survival Shooter with some basic tasks to perform between each wave.
Random events, searching for junk, repairing & upgrading your base, upgrading your gun, discovering intel & medals.
There is a story line and the creatures that attack you get tougher and bigger as it goes on so you have to upgrade or die.
Every so often you back off to reveal a new scene and possibly new enemies!
It saves your progress so you can continue at your leisure.