Old Town: Flags 16-25
Go to the north safe house and look for a pair of towers one block away. Use your climbing skills to go up the towers and feel free to use the grappling hook as well. The flag is at the very top of the north tower.
Skinny Tower, East
Go to the east side of the map and look for a skinny tower isolated in an area with very few tall buildings. Use every side of the tower to climb and navigate your way to the top of the tower. You can also use the grappling hook (assuming you unlocked it) to assist your ascent.
Head to the centre of the map and look for the tower one block away from the central safehouse. The flag is at the top of the tower under the domed top.
Tower Above Sewer Entrance
This can be best reached when you first enter Old Town via the sewer entrance. Climb to the rooftop and look for two towers nearby. The flag is on the north tower.
Head to the northwest corner of the map and look for a lone tower. The flag is on top of the tower.
Radio antenna, South
Travel to the southernmost end of the map (on the west half of Old Town). Travel along the rooftops and look for a radio antenna. Climb the antenna to find the flag.
Southwest Safe House
Starting at the southwest safe house, go to the roof. Climb the radio tower to reach the flag. Note that this is best accessed using the grappling hook.