General Gameplay Hints
Grab everything that's not nailed down, no matter how useless it might appear on first glance. Many puzzles require you to combine items from your inventory, and you'll create new items from these odds and ends, such as grappling hooks and flower bouquets, that are key to advancing beyond your current location. Listen to Sylvie's comments as you attempt to solve a puzzle -- her dialogue will give you an indication of whether you're on the verge of a solution or headed in the wrong direction entirely.
La Valetta: Oliver's Study
Click on the wood paneling next to the end table at the far end of the study to unearth the study's secret panel. To unlock the panel, enter the hallway through the door to your right, then click on the shelf on the left wall to shift Sylvie's focus to a vase. The vase has five segments you can rotate with a mouse click. Rotate each section until you see a date and a Latin inscription; Sylvie will tell you when you've correctly aligned the segments. Return to the study. You'll discover that even though the panel's unlocked, it's still stuck fast. Use the scooter key in Sylvie's inventory to pry the panel open. You'll find a book and a letter from Uncle Oliver that will give you a new person of interest to approach.
Istanbul: Luring Away the Museum Guard
In Istanbul, you'll need to lure the guard from the museum's rear entrance before you can break into its warehouse. Pick up the poker on the asphalt behind you, then click on the archway to your left. You'll find a garbage can with a ball amongst the other trash. Use the poker to work out the ball. Return to the museum entrance and click on the curator's convertible. Wedge the ball under the car's left front tire. The cat in the passenger seat will swipe at you if you reach into the car, so place the fish from your inventory onto the seat. The cat will hiss and prance, then bolt out of the vehicle. Put the convertible into gear, puncture the ball with the poker, then run back to the museum's rear entrance. You'll hear the car crash and the guard will leave to investigate the ruckus, leaving you free to sneak into the museum.
Rome: Escape from the Vatican
As you approach the game's climax, you'll find yourself trapped inside a van in the middle of a courtyard within the Vatican. Use the chisel in your inventory to pry open the doors and approach the archway in the courtyard's rear. You'll eavesdrop on a conversation concerning the stolen artifacts. There's a rusty gate on the courtyard's right, but you need something to batter it open. Approach the boarded window to the van's left and use the hammer to pry loose a plank. Take the broom that's beside the window, then go to the potted plant in the courtyard's lower left corner and grab a handful of soil. Go to the fountain in the courtyard's center and turn on the hose, then click on the stone column fragment beside the fountain. Drop the soil in the water from the hose, then use the broom on the mud to create a slick surface. Use the plank on the column fragment to create a lever, then click once more on the plank to roll the column through the gate. You've now got a large enough opening to escape.