Gobwin Knob has 4 defensive zones:
|Airspace - only entered by flying units, subject to ranged attack by units in the city.
|Tunnels - only entered by light units, some exceptions.
|Outer Walls - must be breached by siege. The more units defending, the harder it is to breach. No breach? Then you can't get in. Hm.
|Garrison - can only be attacked with total control over one of the other zones. If the zone you attack from is outer walls, then garrison walls must also be breached.
City "falls" when all units in the garrison are croaked or captured. Each zone has its own rules of engagement, but they're all considered parts of the city, so we can deploy and redeploy units without expending move. (I'm the only one that gets tired moving around, apparently.)
Ansom's plan is pretty obvious. I hope. He moved his own troops into the tunnels (we let him). Next turn, he'll pull up the rest of his column and hit the outer walls first, to make me keep my guys out there.
If I don't, then the outer walls go down easy and he takes the garrison from there. If I do, then he hits my weak garrison from the tunnels. So he thinks he's got me in a lose-lose, because he thinks I won't do anything on my turn but dig in.