Tattoo
To Hades the wall was a symbol of power and freedom both. As he solidified his rule, accepted more workers into the fold, the thought came that he should commemorate his commitment to Hadestown in a tangible way. And what better way than a tattoo, a permanent mark on one's being?
He toyed with grandiose designs at first, but found them lacking in substance. Hadestown was his kingdom of earth and dirt and rust, founded by his own two hands, and Hades needed something to reflect both hard work and stability.
After giving the matter more thought, he realized that in the end, the most obvious solution was best. Hades marked his arm with the symbol of the wall he'd started and his workers would continue until the end of time. Like the wall itself, it would never fade, never be destroyed, and would remain for all time.
Hades knew that no matter what came, the wall tattoo would remind him of all he'd hoped and worked for.