Below are the different strategies against the Rat.
The Rat's numerous attacks may prove troublesome early on, when the player has not yet garnered enough troops. Having purely archers does not work out, as athey cannot inflict sufficient damage in a short time and they are easily weakened by the javelins of men-at-arms.
Use melee units near the stairs or chokepoints to hold up the Rat's attacks, giving time for your units on your towers and walls to kill the enemy. Armored troops are great for this purpose, particularly Arabian swordsmen and pikemen. Also guard your outlying facilities and farms well, since the Rat's troops tend to head for these buildings.
In a siege camp attack, the Rat cannot do anything most of the time except for catapults, which can destroy wall segments, knocking off any troops. It is best to construct multiple layers of walls to draw catapult fire, or station your units entirely on gatehouses and towers and keep them repaired.
The Rat's castles are very easy to break and they do not station huge garrisons, except around the Lord. The sporadic archer and man-at-arms defenses are easy to overwhelm with superior numbers.
Like with his previous incarnation, the Rat is easily blunted as you can chew through the defenses with lots of ranged units and armored units. Encircling masses of horse archers around the settlement can also work, however you can expect casualties from javelins and arrows. The Rat will keep recruiting units, so it is vital to dispose of his barracks first with either siege engines or from your foot soldiers.
The Rat's keep can be annoying to conquer, as he tends to station a huge number of soldiers there. Use a large number of archers to kill off the bodyguards before taking on the Lord himself.
Sometimes the Rat might come out of his keep when you rampage his walls and destroy buildings. Make use of this opportunity and quickly concentrate your forces around him for a quick victory.