You need to get Salim's thief costume back, but the shopkeeper only wants flawless gems as payment. You need to gather three of them, which are embedded in statues on the city's walls. Start by going after the one on the right side of the map. Get onto the roof of the building nearby, then use the spire jump and flagpole to grab the gem.
The next one is on the left side of the map, on the pier. Use the trampoline to bounce up to the first statue, then swing over to the second with the gem. For the last one, use the trampoline to bounce onto the cliff, then jump and swing your way over to the statue with the gem. Go to the shop to get the Thief Costume.
Go to the gate and stand on top of the switch, then press R2 to slow down time to get underneath the gate. As it turns out, Le Paradox is meeting up with Miss Decibel right now, in front of you! Bentley tells you to tail them, but you use the Thief Costume's sword to break the gear to disable the sword trap. Keep to the rooftops and keep following them, until you need to equip the Thief Costume and go under another gate, then destroy another gear. Go inside the tent at the end to finish the mission.
