Furthermore, as noted here:
http://www.giantitp.com/comics/erf0006.html
The Queen had to deplete the treasury beforehand, or it would be captured along with the city.
And as noted here:
http://www.giantitp.com/comics/erf0110.html
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.