To create your own levels, install Thief Gold to your system (full install), then goto the goodies folder on the CD and unzip DromEd to your Thief folder. Goto it, and you can create your own levels, edit levels, and play fan missions!

By pressing, CTRL+ALT+SHIFT+END while playing a mission you will be instantly transported to the next level.

It should be noted that by doing this trick, you will no recieve any money from the mission that you skipped.