This is mission 1 in the Path of War, in Stronghold 2.
A noble lord, introduced as Sir William, is running down to an island, being chased by enemy soldiers. The lord of the castle instructs his men to bring him down, but a badly catapulted stone destroys the bridge just after William has passed, leaving the chasers stuck in the other side. The lord, known as Lord Barclay gets angry and kills his mangonel handler with his crossbow.
The easiest mission in the game, just send your units to the walled-off area with a boat. They will kill all enemies (spearmen) there, and you're done. Do not forget to set the two Lords to aggressive behaviour or they will only fight if attacked. But even if you do forget it, it should be no problem to dispose of the attackers.
The two intruders leave Barclay's castle. Meanwhile, Barclay reveals that they are searching for the King.