First place you can do it is on the second island, due the upgrades needed on the town.
Enable the upgrade from it putting coins on the weird big squared structure and put more coins on the base camp / town.
You will enable the knights (speaking about the classic scenario). On the 2nd island there will be at least one portal on each side. after expanding recruit 2 peasants to one of the sides and they will convert to knights. One of each will lead up to three knight archers. From the leftmost or rightmost of your kingdom you will have a banner beyond your last / first wall. Put 4 coins for each knight and they will run into the portal to attack it.
The knight will face forward to it slashing with the sword any greed that comes out, and the archers will stay back a bit shooting arrows to it.
Don't forget you can "shield" the knights giving them up to three coins, so instead of unconverting him when hit, it will lose one coin.
Of course you can drop coins between the portal and the knights to focus the greed on them, and also give the knights one back if they lose when being attacked.