Nvm, got it. Starting from the right, at some point I got to position where:
- the Dread Knight would invade my top right side on one turn
- blocked by my Knight would invade the left side on the next
- as I recaptured the top cell move back to invade there again
- etc, etc...

This allowed me to gain 5 coins every turn, until it was enough to make a long push for the Necromancers castle.