First Defence strategy

[ughgh]

Pretty obvious to just cade the left tower
Arrows = enemy direction
Stars = barrel placements

The front left window cade is entirely optional in the earlier waves, but does help against enemies with throwing weapons.
Enemies rarely come up from the right ramp - archers can also stand there to pick off enemies attacking the right side of the cade
Note that the back windows all have barriers - projectiles can get stuck both ways if you try shooting at archers in spawn. Only the far right window lets you shoot at rangers.

examples