Once you start playing, you won't be able to stop
All you do in the game is place your defense towers, whether they be regular turret guns, laser installments, flame-towers, cannons, etc., and you put them in designated areas (you can't just put them anywhere). That's where the strategy comes in. You need to figure out where the best space is that is openly available for a tower in order to keep the enemies from stealing your core pieces from your main tower.
Since the demo is free, you just need to go check it out yourself and you will instantly understand why I just downloaded the game today and have already put over 10 hours into it. You won't be disappointed.