Once You Unlock Iron - (Island 4) You Can Upgrade You Castle Again and Build a Bomb.
-Make Sure All Other Portals between you can the cliff portal are broken.
-Make Sure You Have A Full Gold Pouch
The Bomb
-Up To 4 Builders Can Move the Bomb.
-It Will Start Moving Towards The Cliff Portal as soon as a builder grabs it.
The Knights and Their archers will begin to follow you, as you rush to the cliff portal.
(In Previous Games, You just broke it like it was a Normal Portal)
- When The Bomb Arrives Pay 5 Gold To Start the Incursion.
The Incursion
Battle Your Way To The Back Of The Cliff Portal.
- Use your knights as human shields.
- Pay 5 Gold To Light The Fuse When You Get To The Back of the Cavern
RUN!
Do This On Four Islands, and the Achievement is Yours!
GL!