Go to the grid located there - climb it to reach the ledge on the left.
This way you will get to the balcony with the next two opponents.
You don’t have to sneak up on them.
Attack the enemy on the right, and your friend will kill the one on the left.
You will reach another enemy.
Wait for a moment, and when he turns around, run up and finish him off.
Then, jump over the barrier (the one on the photo on the right).
Once at the bottom, quickly run to the left (remember to move undetected).
You will encounter the last enemy in this room[secret 4].
Now you could go back to your partner - he is waiting near the next gate.
Opening it will be a bit difficult - you’re gonna wanna suspend the alarm first.
Go to another open space with several opponents.
This time, however, you don’t get separated.
There you will meet a new opponent.
The second stands next to the fountain[secret 5].
You shouldn’t have any problems with eliminating him.
Don’t go anywhere, just look around.
You should see the next guard standing with his back to the railing.
Sneak up on him and throw him over the railing.
Flynn will take care of the other guard.
Do this and follow the partner.

If you want, you’re free to finish off the remaining enemies in the area.
It’s not necessary, but it will help you to unlock a few trophies.
Go through the roof, move on the window and ledge to reach the next closed room.
This triggers a cut-scene during which you get a gun with sleeping darts.
Make your way to the central part of the room and eliminate one of the opponents.
But remember that the weapon has a small range, so you should probably get closer.
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