Across Sky Bridge lies the great Castle Solvano.  It is considered the strongest fortress in all the lands, and rightfully so.

You're caravan could be warned; but I find it unlikely.  The Caravan would, however, notice the large enemy force if they tried to arrive at the current Castle First Reach, and most likely would have been slaughtered by now.  If you want, you can write IC expalining how your caravan arrived in the days before the enemy arrived.  I'll leave you with some leeway in this aspect, as it is your story to tell :>