I won! Nice game and interpretation of the limitation.
I found that the speed upgrades didn't seem that useful to me as I was able to spawn so many units that they would slow down on corners/by towers anyway - but that may just be my perception.
Kudos for having so many entities on screen at once, the game ran very well.
I do think it was quite easy as the units are free and I can just keep spawning them, but still very fun.
Nice work!