To explore the unfinished training area in the Parisian Back Streets level, first activate debug keys using one of the methods described on the previous page. Or, download a PC savegame with Lara inside the secret area. (Sorry, but so far I only have a save for the v52 patched game.) Instructions for downloading and installing savegame files are here. PlayStation 2 users can try the alternate method below.
A brief "walkthrough" of the secret area follows. Since it's unfinished, this is just my take on how it might have worked. If you have other ideas, or find anything I've missed, please let me know.
If you like, you can de-activate the cheat on the other side (by pressing Action) and explore as you would normally. The door you just passed through opens from the alley side, so you can return to the start (below) whether or not you have the fly cheat on.
There are two wrought-iron gates at the end of the alley. (These can be seen in the screenshot above.) Like most doors here, Lara can walk right through these. (You can use the flight cheat to get through other solid doors if you like, but you'll find mostly blackness or incomplete textures inside.) If you go through the left gate, you'll come to the end of the level and miss a whole section of the street. The designers probably intended this gate to be locked.
Instead, go through the gate on the right. If you actually use the Courtyard Key to unlock and then open the gate (stand next to it so the hand icon is visible, press Action to use the key, step back, step forward to reactivate the hand icon, and then press Action again to open the gate), it sets up the next area to include a police car and gendarme. If you pass through the gate without opening it, there is no car and the gendarme appears later when you approach the next corner.