Spoilers for the existence of Castiel. Castiel and memories of creation.
Paper fascinates him; when he last touched the world with human fingers, he'd walked in the body of a temple musician in the city of Memphis. He'd lingered in the storerooms, cool even in the heavy humidity of an Egyptian summer, to study the massive rolls, watched the scribes with their pens and many-colored inks, knelt in the temple and raised her flute-clear voice in worship with ink-stained fingers from an afternoon on the banks of the Nile, learning the fusion of art and language with human hands.
Memphis was ancient long before he traveled it on human feet, a city built by millennia of Egyptian pharaohs, rising and falling and rising again. No one who walks the earth can speak Egyptian or remember the hieroglyphs formed by the first people who huddled on the banks of the Nile and prayed for the rising of the river, but Castiel can still sing her song of worship for the pantheon of Egyptian gods and remembers how her hands shaped the language of men on fragile paper the year that Cleopatra was anointed Pharaoh, and the beginning of the end of an empire
His new vessel leaves paper in vast quantities everywhere Castiel looks; filling folders in file cabinets, spread across his desk, fine pencil lines interpreting angle and force, geometry born on the banks of the Nile to measure Inundation, expanding mathematics as humans learned to understand the language of God.
He's an architect, Castiel knows; Castiel's eyes flicker over dimensions and notations, tracing designs that descend from a single woman and a single man in exile, mixing mud and straw when they stood beneath the sky and realized that there were some things that could only be expiated by death.
When he leaves, nothing has been moved from that first evening he came to this man and touched his soul, but he carries the memory of glass and steel rising toward the sky and the vastness of a single human's act of creation with him. It reminds him why he fights.