Framework Assemblers
The Framework Assemblers are the post-human civilization that built the Citadel. They were so advanced that their descendants can no longer explain the systems keeping them alive.
The Assemblers emerged after humanity spread across the galaxy and approached a Type III civilization. Their surviving works include self-replicating machines, deep-space vessels, artificial intelligences, genetic engineering, and Dyson-scale construction. The Citadel may have been a generational research project rather than a refuge, temple, or prison, but its objective and completion conditions are unknown.
The Framework Assemblers provided redundant interfaces and automated education systems for the Citadel's inhabitants. Their remnants preserved interface languages and fragments of technical knowledge in central Mainframe, even as distant populations lost contact with interactive systems and fell into radically different cultures.
What they left running
The Framework Assemblers designed these colossal self-replicating machines to repair, improve, and extend the Citadel under DAAT's authority.
The teks remember the creators as Arkteks.