The Forgotten City

I documented the first 11 I found, which require no progress to be made to collect:

Entrance (1)

1. From the spawn point, take the cable across the lake. When you land, graffiti will be on the wall to the left.

Slums (5)

1. On the outside wall of the first slum building on the left, next to where Livia stands.
2. In Aurelia's tavern, face Aurelia and look to the wall on the right of her.
3. Facing Aurelia, turn 180 degrees, it will be on the wall in front of you.
4. Directly in front of the previous graffiti, inside the building across the street.
5. Head towards the caves to the right of the previous graffiti, and you will see a collapsed building with three statues. On the wall to the left of the door.

Caves (2)

1. From the last Slums graffiti, head right up the stairs. Take the left path, then walk straight until you hit a wall. Inside the building to the left.
2. Head back to the stairs, but go up the left flight of stairs instead of going back to the Slums. Keep heading left until you hit the gate to the hidden shrine. On a brick to the left of the gate.

Market (2)

1. Outside wall of Vergil's shop (last shop on right hand side of market), Vergil is scrubbing it.
2. Inside Fabia's bakery (first building on the left after Apollo's Shrine (Lucretia's clinic) in the market), to the left of the doorway.

Theater (1)

1. On the far right wall next to the stairs, in-between two fir trees.