I was DMing my group through this about 2 weeks ago. They were resting in the keep when the orcs arrived, and they had left the drawbridge down so the orcs just walked right in. I did not bring in the elves, nor was I going to because that seemed like such a cheesy part of the encounter.
"Hey! I know we just showed up out of the blue at the right time, and now that we saved you, we're leaving just like that!"
However, the party did well using the keep as a defensible position and beat back all of the orcs. Then again, my players are old school players, so they are frequent users of things like flaming oil and using the environment to their advantage rather than rely on spells for things like damage and crowd control.