Looks and feels great. Might need some balancing. It would be nice to get some indication of which direction the enemies will be coming from.
I would balance this out by putting a unit cap in, which can only be raised by building farms. Puts the breaks on and makes you have to find the right balance. Might help with the lag at later stages, as well. Maybe emphasize upgrading units instead of making more of them, also.
Really good submission.