Grid Forge TD is an incremental tower defense game. 

Tower defense

Enemies start at one side. Your job is to stop them reaching the other side. You can place walls, towers, and buy upgrades to achieve this.

Maze building

You place tetronimoes to block the enemy and build up the path that they must follow. You have limited space to work with (at least at the beginning...) so smart placement matters. 

Tiles can be stacked which provide bonuses to the towers on top of them. This leads to the question - do you make the path longer with this piece you've been waiting for or do you build up your walls for bigger bonuses?

Meta progression

With each run you earn research points to buy upgrades. These can be everything from making towers do more damage, to generating tetronimoes faster, to increasing the amount of gold you start with

Prestige

Once you defeat wave 12 you unlock the next prestige level. You'll face tougher enemies, but you'll also earn more resources and be able to buy new upgrades like ice and lightning towers.