Enemies walk a path, you place towers beside it, and the wave counter climbs. The elements are the reason to keep watching: a tower on its own does an honest amount of damage, and a tower next to the right neighbour does something else entirely, so the layout is a chemistry problem rather than a question of how many turrets fit.
There is a speed control for when you are confident and a next wave button for when you are impatient, and both of those are traps in the good sense. Twenty waves is short enough to attempt a perfect run and long enough that a bad opening will still be costing you at wave sixteen. It was built for a thirteen kilobyte competition and there is nothing in it that is not load bearing.