The Towerborn are a type of servant of The Manufactured Newborn.
Similar to how the Newborn assimilates organic and inorganic materials, it can also enter and assimilate a person's body, restructuring it, replacing organs and bones with metal and clockwork. The Towerborn also have a limited ability to assimilate matter into themselves, similar to their master.
Paul from one thousand and one nightmares is a failed Towerborn. The Newborn which infected him was damaged in some way and it could no longer grow.
The first full Towerborn we see is Frasier Sweeney of Cobbled Together. Somehow, he was able to regain his original personality back, though, and seeks revenge against the Newborn. A Towerborn was also seen by one of The Free Radical's operatives and the Faceless Bastard worked with one to stop a piece of The Dying Man.
Stripes Cat of Born Of Data eventually made the decision to be converted into a Towerborn rather than go insane, but only half his body was modified. However his whole head was kept intact as a mockery of the human race.