Alrighty, been considering my options for Thorn. Let me get some outside opinions to help me decide.
1) Thorn, suspecting some manner of treachery, organizes a defense force within the city as well. The idea being that it doesn't do much good for her to impersonate the Queen if she stops impersonating her at the first sign of trouble...and additionally, that magical means like teleportation rituals could send small teams of elite troops into the city, bypassing the pass completely.
2) Thorn, in guise and regalia of Queen, goes to the pass to rally the troops there, and perhaps draw out more assassins for us to try to kill before they can kill Thorn.
3) Thorn assumes new disguise and goes to the pass to help fight, leaving the palace empty of a queen.
I've listed the options roughly in order of the degree to which I'm inclined to do them. The problem is that 1) feels the most like what Thorn would actually do...but it runs the risk of taking her out of a combat where she may be needed, which hurts the party, and it hurts her personally since she misses out on exp and rewards. 2) is an acceptable compromise, though her ability to fight while simultaneously pretending to be the Queen will be curtailed a bit. 3) is mechanically the easiest option, but it is narratively inexplicable, and completely nullifies (IMO) the point of Thorn ever having pretended to be the Queen in the first place.
Thoughts?