Defense Grid is a tower defence game where waves of aliens spawn in and head towards your power source. They take a power core, then head to the exit. Sometimes the exit is the entrance, but other times there is a dedicated exit. Some maps even have multiple entrances/exits, so essentially you may be fighting two different battles. The aliens never attack your towers; they are only concerned with stealing your power cores. You lose if all your power cores are taken. When aliens are defeated, the power cores slowly return to the source.
You can place towers down where there is a slot for them, and there are usually plenty of slots. On some maps, you may only cover 30% of it. This gives you plenty of decisions to make to get the most efficient use out of them.
Some missions have a set path, and you can place towers to the side of the path. Others have many square areas, and you end up placing towers in a maze structure to slow the aliens down and do the most damage. There must be a path leading from their entrance all the way to your power source; you must give the aliens a fair chance of winning!
There are many towers to place, with many of them being introduced over several levels. Each tower has a few attributes; attack power, range, attack speed, and may be better suited against certain alien species, or against individuals, or groups of them. Gun towers are your basic machine gun towers; Cannon towers do larger damage, have longer range but have slow fire rate; Tesla towers are great against shielded enemies; Temporal towers slow enemy movement; Inferno and Concussion towers are good against large groups of enemies since they damage multiple enemies.
On many levels, you will be attacked by flying enemies. Your standard Gun towers can attack them but aren’t effective against the stronger ones. Missile towers are highly effective but only attack the flying enemies.
You gain credits by defeating enemies. You also accumulate interest on your credits to encourage holding off on your purchases.
After a certain mission, you can call in a rechargeable, focussed missile strike. This is great as a last resort if a strong enemy is about to leave the map with a few power cores. However, you don’t acquire credits for defeating enemies this way.
The game starts off easy enough, I was just placing a tower everywhere I could. Later on, you need to make sure you upgrade the towers because the basic ones just don't cut it. Some levels I really had to retry many times and come up with a new strategy, especially on the DLC missions which are really tough.
I played Defense Grid 2 a year or so ago, and I think this game might actually be better. It’s really addictive and can easily play for hours at a time. There’s plenty of content and you can replay missions to beat your previous score, or try the Challenge missions which have different rules.