Cool idea, Its hard to actually build due to the rounds starting instantly after the last enemy goes down, so I end up just disabling the last enemy so he cant hit me and just put together a fortress and then start the next rounds. I guess its supposed to be difficult to put stuff together but the parts are simple so not a lot of planning is needed.
Doing it the way I did it did make it more boring.
I feel like its cool though and would benefit from being given a larger map. So adding more mobility is more useful and you can hide from enemies to add attachements while still being under threat because the enemies are still approaching.