I had reached a dungeon level around 10-15 floors down, when I returned to Torchlight (the town) via a Town Portal scroll and got a quest from the Trill-bot to kill a monster on level 3. Easy! I ran down to level 3, one-shotted the baddie and instead of clicking an Identify scroll I clicked a Town Portal scroll. I was hoping it wouldn't overwrite my previous portal if I didn't use it, so I ran back to my original portal but it brought me back to level 3!
So have I really lost all my progress? Can I not easily return to the farthest level I've reached?