You can place a (6DU) Buffbeam on the small bridge; there is a small buildable area marked with hint 2.
Characters can be stationed under the bridge, where the crate also spawns; the entrances to the bridge can be secured with Reflect Beams as needed (sometimes a goblincopter or a rocket will explode, but the defenses won't be damaged).
If the defenses take damage, place a Golem in addition to the Shroom Spore Tower
Semi-AFK-friendly; auras need to be repaired every few waves