Thief: The Dark Project Cheats

Proceed through the training section until reaching the courtyard. Fight with the guard until he retreats, then hit him until he stops. Take the key from the table and enter the area where the guard originally appeared. Move to the end of the hall and use the key on the door. Then, enter the door to reach a court containing a ball and scroll.

If you are fed up with the guards and you want to explore
properly without being caught, use the following trick. From the
beginning, enter through the gate and look around. Try to upset
as many as guards you can by going in secret rooms (ones with
red hammer symbols by door) and getting the guards to raise the
alarm. An increasing number of guards will follow you the more
you go around the house in circles. Save your game once you have
done this, as you may end up in a dead end. Once you think you
have got enough on your trail, try to find the entrance (the way
that you came in) and go out. Jump down the side streets (the
pits on either side of the door) and shoot up your rope arrow.
Wait until all the guards run off down the path to search for
you. It will take about thirty seconds for them to get to you.
Quickly go up the rope arrow and jump back onto the bridge. While
they are down there, you will have no problem getting back inside
before they reach you. Go back in and pull down the lever on the
right as you enter. The door will close and you will have successfully
locked them outside. There may be a few guards remaining. Go around
and kill them. Save your game after killing each guard. Places
where guards may possibly still be are: in the chapel; at the
very bottom floor (where the coffins are located); and on the
floor where the bedrooms are located (first floor down the stairs).
Once you believe that you have killed every guard, you will be
free to carry on with your mission and find everything in peace.
Note: There could be guards there that you may not have noticed
for a long time; be on your guard and keep your sword drawn.

all you have to do is hold the shift button, the alt button, the ctrl button and the end button to skip levels

Press Alt+Ctrl+Shift+End And you will skip the current Level.
NOTE: This only works with the Version 1.33 Patch installed.