Hi! Nice attempt. I think there is a working idea here, but there is a lot to be desired.
There were some problems with my play through - first of all, the build phase should have a longer time - at least for the first round or should end when the player decides to end it. A little walk through would have benefited greatly - but I know in the limited amount of time, that can be hard to implement.
I didn't know what exactly I was doing, and the defend phase started. The ant got to my crystal after I started spawning mechs and they were not able to defend from that distance.. perhaps giving the player the ability to defend - like a gun or something would be nice, or making mech build spots closer to the crystal would work.
I found the artwork to be inconsistent and, yeah, it's clear that you were using asset packs. That's not a bad thing, but the goal, imo, when using texture packs is to make sure that the artwork blends together -whether that requires editing or whatever. The mech design art quality clearly differed from the asset pack - and that was off putting for me.
Also, the resolution was too small - it looks like you kept it at the 640x360 resolution, and full screen did not work correctly.
Anyways, it would be cool to see this idea expanded upon and polished more.
Good luck!