Yeah, those last rounds were meant to spawn basically all together and try to rush your towers, so you needed towers along the entire path or some of them would get through. But I messed something up when tweaking the campaign timeline and now the last couple of waves are super slow and easy. So you just have to wait those out if you want to see your score. The lesson here is: even tiny last-minute changes should be tested properly. 😄
I don't know yet if I'll continue working on this game, but if people like it then I might expand the campaign and add some more levels.