This level is very easy. You face 4 rat lords, each starting with 2000 gold while you have 4000. There is two ways to kill them. One is a longer way, and one is short.
THE FIRST SOLUTION: (LONGER WAY)
The rat's early attacks are a nuiscence, and you can defend them by buying 10-20 arabian archers to defend you farms and castle. Then, build up a steady economy, and once youve got a big army, (about 30 crossbows, 30 archers, 15 swordsmen and anything else you'll like.) Kill the closest rat,get a bigger army, attack any rat, and keep going like this. Remember, if you attack the second rat, get an army of about 150 men (at least 60 crossbows and 45 swordsmen). This might be a overkill but you can wipe out all three rats at once like that.
SECOND SOLUTION: (FASTER WAY)
Get a granary, get a market, and get a mercernary post. You have to do it fast. Sell your goods at the market, and spam assassins. Send about 25 assassins at the rat south of you. Then, train 30 assasins and send them at the rat southwest. Continue making assasins and work your way up. The rats should be dead in 5-10 minutes.
ALTERNATIVE : (THE FASTEST ONE)
You must take moves very fast.
Place a granary near the oasis, and place four apple farms next to it. After finishing it, you can leave your economy alone.
Build two houses to make it quick for training soliders. The final step is to site a mercenary post to train Arabian swordmen. With your starting gold, you can get around forty ones. As you get 20 units, set them at attacking position and dirctt them climb the southern Rat's keep.
At the same time, you cantt ignore your sowrdmen training. When you run out of your gold, you should get another twenty, do the same thing to them as the former ones. Send them southwest to finish another Rat's keep, halfway the first group shall complete their jobs. Send the men southwest too, to cooperate with the second group.
While marching, return to your castle to train more Arabian swordmen using the money from Rat. Depleting your money, send all of them southwest. At this time, the southwestern Rat's shall be destroyed, and your men are still enough to take down the easteren keep. Gathering your men at the eastern keep. Then, march to the eastnorthern keep, the final one, it's Rat's end.
Igore the defending troops. Hit the walls directly. Its's harmless because the archers take few points to the swordmen.
This way could take less than five minutes.