Defend the trenches with towers
When a wave approaches, look at the trench route toward Spiridun. Preview tower placements on higher ground and check which stretches of the trench fall within range. If you tried to order soldiers down a cliff into the trench, switch back to tower placement to handle that lane.
Use troops on the islands
Send your heroes and troops after units on the islands. Securing territory opens more room for your settlement and defenses. Treat the island fight and the incoming trench wave as separate jobs when deciding where to act first.
Recognize the end of wave 6
On Forgotten Powers 0.19.4e, a run on seed 383977 reached wave 6 with several towers around the home island and 89/100 base health. The Runestone selection screen appeared at the end of wave 6. After choosing a Runestone, the player returned to the map and the HUD showed Wave 7.
Check the wave counter when you return to the map. Tower sites and starting resources can vary with the run, so use the terrain around your own trench lane to choose positions.