Suddenly I was defeated. There is a special loose condition, "Troops killed: 30" nobody tells you about. Defiantly I started anew, but to my rules. Bought in the food types I could not buy at the market. Smashed the inner circle of walls and set up a huge economy, processing wheat and iron. Had double rations and -24 taxes :). All my melee troops I let retreat onto the gates and my outpost I abandoned. The invasions did not bother me any longer. Waited until the necessary amount of gold was achieved.
In general, this map (which answers to the name "Heaven in the desert") is easy. Beginners may have fun and may profit playing it. It's the restrictions at the market that make the mission somewhat interesting. You cannot buy or sell away the wood and thus must household with the stockpile tiles.