The Rat's numerous attacks may prove troublesome early on, when the player has not yet garnered enough troops. Use a stationary group of melee units near the Rat's attack points attacks, while ranged units atop towers take shots against the invading infantry. Simply building up the defenses at the area can take away much of the Rat's later attack potency.
The Rat's castles are very easy to break with their thin walls and sporadic garrisons. The defenders can be overwhelmed with superior numbers, which tend to be reinforced quickly from the barracks. Use masses of horse archers encircling around the settlement, or armored units to wear down the outer layers, then the final group around the lord.