Originally Posted by Iago
Furthermore, as noted here:
The Queen had to deplete the treasury beforehand, or it would be captured along with the city.
And as noted here:
Field units disband/disappear, but city Garrison forces freeze in time, and are thus susceptible to capture (along with any other resources belonging to the city). Thus, by first depleting the treasury and then using it to deplete the Garrison, the Queen double-denied GW's side.
Awesome Catch. I thought she had sent the troops out so they could link up with Jetstone but now it makes perfect sense.