Archie, a small computer-based library of architectural design cases, is described. Archie helps architects in the high-level task of conceptual design as opposed to low-level tasks such as drawing and drafting, numerical calculations, and constraint propagation. Archie goes beyond supporting architects in design proposal and critiquing. It acts as a shared external memory that supports two kinds of design collaboration. First, by including enough knowledge about the goals, plans, outcomes, and lessons of past cases, it lets the designer access the work of previous architects. Second, by providing access to the perspectives of domain experts via the domain models, Archie helps architects anticipate and accommodate experts' views on evolving designs. The lessons learned about building large case-based systems to support real-world decision making in developing Archie are discussed.