Architectural sketching and massing are used by designers to analyze and explore the design space of buildings. This paper describes a novel multi-touch interface for fast architectural sketching and massing of tall buildings. It incorporates a family of multi-touch gestures, enabling one to quickly sketch the 2D contour of a base floor plan and extrude it to model a building with multi-floor structures. Further, it provides a set of gestures to users: select and edit a range of floors; scale contours of a building; copy, paste, and rotate a building, i.e., create a twisted structure; edit profile curves of a building's profile; and collapse and remove a selected range of floors. The multi-touch system also allows users to apply textures or geometric facades to the building, and to compare different designs side-by-side. To guide the design process, we describe interactions with a domain expert, a practicing architect. The final interface is evaluated by architects and students in an architecture Dept., which demonstrates that the system allows rapid conceptual design and massing of novel multi-story building structures.