The quest is the last one in the challenge series in Argos when you must defend the town center against enemy siege for 30 minutes. You get it around 28-32.The challenge is not hard and pretty easy in coop. The map helps alot, since you are kinda surrounded by natural walls and the enemy can come only trough small pathways where you can kill them easy.
You start in the center of the map with 3 pathways from where your enemy can come W-west N-north and E-east(this assuming you're doing in coop, if not, they will come trough S-south as well which is a bigger pathway harder to defend; so do it in coop, since yur partner will build there his defences and its much easier this way).
You start with 2000 of each resources and with 5 villagers. Right away move 2 villagers to the W and 2 to the E and build fences, and the last one to the N and do the same and keep there. Age lo level 2 to be able to build towers and as soon as you age, start building them. In the mean time make villagers, till you hit the max limit of 30 (n need to build houses), and send some to the stone mine, some to the trees and some to help you build towers and repiar them and the fences. Build more and more towers and some fortresses, and also a market is quite helpable since you will have at certain times more of a resources that you need than other resources. You could also build an archery to train archers, since the enemy will come sometimes with battering rams and its better to destroy them quick.
Overall its not hard like the other challenges in the game, much easier in coop and its the 4th challenge in the series (first you will have to defend the town in 15 mins, then 20, 25 and the last one, this, in 30 mins, so by time you do this last one, you'll get familiar with the map, strategic points and so on)
Also i think this is the best mission for boosting your level, 55k xp every 30 minutes its very good; and after 2-3 successes on the misiion, you'll find it piece of cake even if done solo.