Below are different strategies against the Snake.
To counter his slave raids, a few groups of archers are useful scattered throughout your industry and farm buildings.
The large numbers of both European archers and Arab bowmen that the Snake sends as part of his main attacks can be sufficient to pose a threat to tower and wall mounted missile troops, especially if the Snake has built up a large overall army of 300 or more troops (which can happen quite often due to most of his troops being cheap to create). Due to their deployment in such large numbers and in loose formation, as well as the fact that advancing spearmen and laddermen will be distracting your troops' attention at the same time, they can take a while to kill using the conventional tower mounted archers. Several crossbowmen, each one of which being able to kill one of the Snake's units in one shot as well as being better armoured), are therefore useful alongside your archers.
With your missile troops distracted by enemy archers, some of the Snake's spearmen and laddermen may have reached your walls. A few slingers amongst the archers/crossbowmen on your walls may prove useful in killing them (as they can fire more accurately directly downwards), but they are rarely much of a threat anyway.
The Snake's towers are small, but are usually packed to full capacity with archers. Trying to kill the lord with mass assassins probably won't work because of this (unless you rush him at the start of the game), although it may be possible if you target him after his army has left his castle on an attack, when he has fewer men on his keep. Instead, after his troops have left the castle on an attack, start the siege by destroying his towers from afar with catapults or trebuchets. This stops as many archers from firing on you during the siege and means that an access route is available into the castle without the bother of trying to dig up the moat. A group of fire ballistae can be used to fire upon the archers and other units on his keep, which can be killed quickly. Finally, as the Snake usually keeps trying to replenish his archers and other troops until the bitter end, his fortress will need to be set on fire to prevent this happening. Destroy the wells with the fire ballistae so he can't put it out. Once the fire stops, a few assassins can then be used to kill the lord.