Start from nothing and place your nexus. Use "WASD" to move the camera, and find the starting position of enemy gates. Build tiles for farming and placing towers. Farms make money based on their adjacent empty tiles, and their type. Create a path to the enemy gates, and defend against the 6 waves of enemies. Be careful, enemy gates left on their own will multiply, but enemy gates will die after spawning 6 waves. You will WIN only when no enemy gates are remaining. Keep an eyes on you starting GOLD, it's not much! (win screen does not exist, if you end up doing it.)
(Don't mind the typo in the image! :D)