Roman Circus of Tarraco, at Tarragona.
 

This building is outstanding, not only because of its architectural achievements, but also because of its place in the city: it is built inside the walls, close to the Comitium, a rare thing because of circus dimensions. Placed on the lower terrace of the higher zone of the city, we can think that was designed with all the Comitium elements, planning in this way communications between buildings behind and the city in front. It was really an "architectural barrier" separating Imperial city -provincial Comitium- and commercial or residencial areas.
Planta hipotética del Circo
Hypothetic plan of the Circus

Vista de los restos de gradas
View of remainings of rows
Urban design dates from Flavian age; its construction is from the end of 1st century, Domitian Emperor age. Therefore, it is the last sample built on Imperial area.
 
It was not exagerately big: it was probably conditioned by its place inside the walls. It was 325 m. long and 105-115 m. wide. At the inner place, the arena, was 290 m. x 75 m. It was divided by a "spina" being 190 m. long. Its capacity was for 25.000 spectators in a cavea with 12 files of seats.
 
General facture of the building was composed by "opus caementicium" at the structural vaults and "opus quadratum" of ashlars at the podium, stairs and façade. Vaults did function as a structure for supporting rows and un upper surface or visorium.
At its western lateral there was a parking for charts carceres and also the "porta Pompae" i.e., the main entry.
Perspectiva seccionada del ángulo sur este del Circo
Sectioned prspective of the South-Eastern angle of the Circus
At the opposite side -the Eastern one- there was a way out for wounded gladiators -porta livitensis- and the general access for spectators to the rows. At its center there was a tribune for judges to the racings. This whole area was supported by radial vaults, that were also used as communication corridors.
 
At the other two lateral, long and straight, there were rows. They were supported by parallel barrel vaults with 40 degrees slope, in order to get a hollow structure.
 
Southern lateral is composed by 46 of these vaults. Over them there were rows. Façade, limited to this southern elevation, was composed by series of archs supported by false pillars semi-detached to walls.
 
Northern side also got them, but, as it was placed on a slope, it was previously excavated and then vaults were built. This way is exclusive to this Circus, since rows were usually built on slopes. Over it, there was a presidential tribune: the pulvinar. This tribune was communicated with Comitium through stairs that made easier access to outstanding spectators.
 
Its useful life was long: almost 300 years until 5th. century. It was probably used up to 7th century under Visigothic domination.
 
This Circus is one of the best preserved on the world, just because it was under new buldings that never destroy Roman structures.