- I had drawn some floor plans for "standard towers". I even knew that there would be a "standard tower dungeon" with barracks, prison, storage and armory, so I could use that as my "Level One".
- I had a random encounter key for woods in the general area. I may have intended for the adventurers to explore the woods on the village side of the river, but I had something to fall back on when they surprised me.
- I had a "local mythology". I knew there was a now-eradicated "old faith", so I knew that the Elder Tower was tied into that old faith, and to its eradication. I could answer questions about the history of the tower, even before I'd consciously worked it out.
So I had something to work with; I wasn't completely in the dark. And not being prepped may be a blessing in disguise, because I intended for this dungeon to be the really tough one in the area. If I'd written everything out, not realizing the players would pick that as their second adventure, it would have been a TPK. Now, I can scale it down from what I was originally planning...
(Actually, although I'd barely started designing the Elder Tower, I did have a few notes already. Including my version of stirges roosting in the upper levels (I don't call them stirges, but it's the same basic idea: bloodsucking evil birds.) Turns out the stirges came close to killing a few party members, but fortunately they've survived, so the adventure can continue when we all come back from vacation.)