My lord Miao Ying was garrisoned in the Turtle Gate city of the Grand Cathay great wall.

My secondary lord Gongming Zhou was right next to the Turtle Gate city, on the Cathay side of the great wall.

On the next turn multiple armies of Chaos Dwarves attacked & instantly began the siege of my Turtle Gate city, but my lord Gongming was not shown as joining in the defense of the city.

I reloaded the previous turn and tried moving Gongming slightly on the campaign map within the zone of Turtle Gate city but it did not make any difference - he was not allowed to join the upcoming defense of the Turtle Gate city. 

Then i tried putting Gongming into the Ambush Stance (he was still next to Turtle Gate on the Cathay side of the wall, as shown in the screenshot) and he was finally allowed to join in the upcoming defense of the Turtle Gatecity.

The enemy AI was not using the "Lightning Strike" skill to prevent Gongming from joining, since the enemy AI was always able to bring all his multiple armies into the siege battle.

Please take a look into this issue, it makes defending the Cathay great wall Gate cities quite tedious.

Thank you.