Another way that Defense Grid 2 stays interesting is with well-designed level layouts. It alternates between grids where tower placement directs the flow of aliens, and more rigid layouts, which made me realize how effective fully upgraded long range missile towers are on seemingly useless tiles.
Flexible difficulty settings and high scores made Defense Grid 2 as difficult as I wanted it to be at any moment, so it never got frustrating or too easy. And no matter what kind of challenge I was looking for, it was always fun to tinker with. I'd tell myself I just wanted to see what the next level looked like, but then I'd start poking at it. I'd figure out how to make the aliens' trip twice as long by blocking a tiny path, or how to get the most out of my flamethrowing inferno towers right where the aliens emerge. Before I knew it, an hour had passed and I was trying to crack wave 22. 2b1af7f3a8