Rules · Cities · Plans · Geometry
Lay out the capital as a circle with four gates at equal intervals, each facing the region its road leads to, and put the ruler’s palace at the centre, so that no part of the city is further from him than any other.
4 gates on the diagonals (Khurasan NE, Basra SE, Kufa SW, Syria NW); palace at the centre
Al-Mansur founded Madinat al-Salam, the City of Peace, in 762 and finished it about 766. The chroniclers tell that he had the plan marked on the ground in lines of ash, walked round it, and then had the lines traced with cotton seeds soaked in naphtha and set alight. The circle was an old form in Mesopotamia and Iran (Parthian Hatra and Sasanian Gur, or Firuzabad, were round), and it suited a ruler who wished to stand at the centre of his state: the four gates, named after Khurasan, Basra, Kufa and Syria, opened towards the four quarters of the empire.
The reported dimensions vary with the cubit used; modern reconstructions give a diameter of roughly two to two and a half kilometres. The plan did not survive its founder for long: within decades the city had spread far beyond the walls, and the caliphs themselves moved out to new palaces on the Tigris.
Draw a pointed arch from two centres on the springing line. With the centres at the springings, the arch is equilateral; with centres beyond them, a lancet; with centres between them, a drop arch. Late Gothic added arches of four centres and the reversed curve of the ogee.
r = s (equilateral) · r > s (lancet) · s⁄2 < r < s (drop)
Classify arches by the geometry of the intrados: the number and position of the centres (one for round and segmental, two for pointed, three or four for basket and Tudor arches), or a single continuous curve (ellipse, parabola, catenary), and distinguish true arches, with voussoirs, from corbelled “arches” of oversailing courses.
Throw an arch diagonally across each upper corner of the square, backed by a small conch or cone; the four squinches and the four walls between them make an octagon, on which a round dome can sit.
octagon side = S (√2 − 1) ≈ 0.414 S; squinch span = octagon side
Orient the prayer hall so that its principal wall, the qibla wall, stands square to the direction of Mecca, and mark the middle of that wall with a niche, the mihrab.
qibla wall ⟂ direction of the Kaaba; mihrab on the axis
Build the congregational mosque as an enclosed courtyard (sahn) with a covered prayer hall (haram) on its qibla side and shaded porticoes (riwaq) on the others.
Make the prayer hall of identical bays of columns and arcades, so that it can be enlarged by adding aisles or bays without changing its character.
hall = n aisles × m bays of one module
Cite this page Chris Paton, “The round city: every quarter equally near the ruler”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/baghdad-round-plan. Text, drawings and models licensed under CC BY-NC 4.0.