The Snake can be a troublesome opponent to fend off early on, due to the fact that his armies can be assembled fast consisting cheap troops, who tend to run in loose formations, distracting your defenders. He also attacks rather fast, necessitating quick defensive measures, such as sealing off the castle early and placing ranged defenders in towers.
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. To counter his slave raids, a few groups of archers are useful scattered throughout your industry and farm buildings.
When it comes to a siege, the Snake's castles feature towers are small, but are usually packed to full capacity with archers. For this particular reason, the siege should be started by destroying his towers from afar with catapults or trebuchets, also giving an available access route into the castle, without the bother of trying to dig up the moat. Try getting the Snake's campfire under fire control in order to stop him from reinforcing his troops.