To the unit spawner:
1. Place a unit spawner
2. Drag select the spawner(Click and hold the left mouse button and drag the rectangle above the spawner, then release the mouse button)
3. A window should pop up.
4. Click the green "+"-Button and select a unit.
5. Add more units or play around with the settings of the window
The game series is not finished yet and maybe I will add ocean/land battles in the future.