Thief Gold is essentially The Dark Project plus three new missions, some enemies and various level fixes, particularly with Bonehoard. Could seem like a light offering, but the new levels actually add quite a lot to the original in both volume (the levels are huge) and storyline. Two of the new missions focus on obtaining the four Talismans needed to open the Haunted Cathedral, while the third is a heist job not related to the rest of the story.