Oh gosh! That was a tough fight! I finally beat them! This is a really fun strategy game. This was my final push before victory!
The trick was to expand in a diagonal direction with cheaper turrets, while balancing reinforcements with walls and upgrading your income! This keeps the enemies away from your "farms" while also letting your innacurate LVL 1 turrets have all the enemies spread out to be hit! Try to get 2nd level turrets ASAP. Then finally you can build up an army to finally push back against the defenders and give yourself a proper chance at winning! Then it's just maintaining your towers until victory!
Really fun game! Good job!