Giles' Cellar Key is used to open Farmer Giles' Portcullis, under his farm.
The key can be obtained in three ways:
- Complete the Cold Comfort Farmer and Blind Date quests, then purchase the farm; the key can be found by searching upstairs.
- Complete the Red Harvest quest and search Giles' grave.
- Do neither of the above quests and on your return from the spire, the farm will have fallen into disrepair. Search Giles' grave to obtain the key.