It depends, did you initiate the battle on the strategic map, that is, did you order the garrison to sally forth?
If so, you have to rout all enemy units in order to win the battle, if you exit it, or let the timer run out, the battle might end as a draw (with the enemy still besieging your castle), or, in the worst case, you lose it.
If, however, the enemy tried to storm the castle, you simply can wait for the time to run out (provided you play with limited time - I always do, for this very reason, among others).
Bookmarks