Out in the yard you will encounter a countless number of baddies. The easiest way to get through this alive seems to be this:
Sneak out to the left and hide behind the lorry. Then run past behind the statue to reach the lorry in the far left corner which was shown to you in the cut scene.
Collect the MP40 and finish as much of the baddies as you can. Finish the rest with the Luger.
When you are done with them return to the lorry where you found the big gun and collect the detonation device.
Use the detonation device at the well.
Jump into it and dive through the passage.
Climb out at the end.
Pull the lever to open a door.
Step through that door and use the lever to activate the secret door.

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