The level's first mask is on the rotating tower just east of the circus area.
This one is on the roof of the tavern in the northern part of the village.
You'll need Sir Galleth for this one. In the northern part of the town is tall building only he can scale using his Catapult Crash technique. This building - directly northeast of the clock tower - has a mask on its roof.
You'll need Sir Galleth for this one too. In the southeast corner of the map is a tower only he can ascend (it's climbed during the "Eye in the Sky" mission). About midway up, there's a plank on the tower's eastern side that holds a mask.
You'll need - you guessed it - Sir Galleth to reach this mask. At the southernmost point of the map is a tower only he can scale (it's also climbed during the "Eye in the Sky" mission). At the top, you'll spot the mask behind some breakable boxes.
This is found during "Juggling Act." Proceed until you pass the first trio of trumpet frogs. After firing the next rope arrow, smash the red box on the platform to reveal a secret stash of arrows. Fire one of these at the platform across the gap to smash the barrel and reveal a target; fire a second to create a way over. Cross to the platform to snag the mask.
Just after the set of four trumpet frogs is a rope. Climb past the first platform to find a second, higher platform where a mask is hiding.
This mask is found during the mission "Mechanical Menace," which means you'll need Sir Galleth yet again. Proceed until you reach the tower inside the cave, and turn right. Below is a very well hidden mask on a ledge.
This one is for Bentley. Proceed through the mission "Shell-Shocked Heart" until you reach the rafters in the blacksmith's shop. Just after the first set of lasers in the first room, this breakable wall be to your left. Clear it with a bomb to grab the mask beyond.
Continue the mission until you hack the second door. In the room beyond are some spinning gears. Hop across them, and toss a bomb onto the crate on the nearby platform. The explosion will destroy the crate and reveal a mask.
Once you've recovered every treasure in the level, the arcade machine in the hideout will be repaired. Boot it up to play Super Alter Ego. Near the start of the second zone is an optional portal on the lower half of the screen. Enter it to find the level's mask at the end of this optional zone.
Note: For more detail on collecting the treasures required of this mask, see our Sly Cooper: Thieves in Time treasure locations guide.