The cylindrical shaped Westhafen Tower marks one of the main entry points into Frankfurt city, and constitutes the landmark of a newly developed urban quarter: the Frankfurt Westhafen. The tower structure is a stack of square floor plates inside a cylindrical glass enclosure. With the optimal ratio of circumference to area of a circle, the cylindrical outer skin minimizes facade area whilst making maximum use of floor space. Superimposing square and circle creates four-storey winter-gardens which rotate by 90° every four floors. These winter-gardens both reduce energy consumption and allow for vertical connection of office units, independent of the commonly accessible circulation. The triangular facade elements can be opened in order to naturally ventilate the offices. When the windows are open, the facade reminds of a birds puffed out plumage. Its many facets create an ever-changing play of light.