So this is what happened. My commander's division entered the field opposite Shaun's horse.

Who scurried out of the way as I advanced towards them.

They had a near miss when the rest of our forces entered the game, but they managed to make it across to the other side to threaten the flank of my newly arrived infantry.
Shaun's infantry mostly formed a strong battleline to hold against my commander's division, whilst a couple of units entered the fields to assist the defenders there. Their commander watched from his horse.
There was missile fire throughout the game, but it didn't prove too game-changing on either side. I was nervous of my cavalry heading straight towards a mass of longbowmen, but it seems these apparent veterans weren't very effective at range! However two arrows did actually hit my commander, but luckily they both glanced off his armour.

I had to reform some of my forces to face the threat of Shaun's cavalry now on my flank, whilst my heaviest infantry unit assaulted the walls which were defended by some seriously outnumbered crossbowmen.... but completely failed to shift them!
Meanwhile my cavalry had charged into Shaun's right flank with infantry close behind.... But it didn't go well for them and they failed to break through.
There was skirmishing around the walls with missiles from both sides. Some of Shaun's advance guard also leapt out from their position to attack a small group of Italian handgunners at close quarters.

Meanwhile on my left, the infantry had come to support the failed cavalry charge but it was too late. Both of my units were broken, and I lost my commander - Shaun's infantry surged forward in pursuit.

I was sure my heavy infantry unit would make it over the wall, but amazingly Shaun's defenders still held, as his commander rode up to offer his support.
However I had some brief satisfaction when my Italian handgunners managed to not only survive being assaulted by some heavily armoured men-at-arms, but managed to beat them back! Shaun's cavalry meanwhile had charged some of my longbowmen and were driving them back towards my remaining unengaged halberds, who were a bit unsure of what to do and remained in place.
However with my left wing broken and my commander dead, things didn't look good. Some Italian crossbowmen were the first to turn and run (with their employer dead, they didn't see much need to stick around!). Shaun's right wing advanced on my flank, and it was looking like I was going to be surrounded as I had failed to break through the walls. Thus I conceded defeat, and my second-in-command negotiated peace terms with the enemy!