Tallboys are City Watch guard units in Dunwall. They consist of a tall, mechanized exo-skeleton which completely encloses the actual guard until they are ready to attack, at which point the armored suit opens up to reveal the guard within, armed with a double longbow. The long legs of the armature keep the guards out of the reach of the plague-infested rats and also provide a better line of sight.
Tallboys can be found guarding Dunwall Tower, inside and out, as well as the Flooded District.