The Context: Why the Flavians Built the Colosseum

The decision to build a great amphitheatre in the heart of Rome was not purely architectural — it was political. When Vespasian, founder of the Flavian dynasty, came to power in 69 AD, Rome was emerging from the civil war of the Year of the Four Emperors and the devastations of Nero's reign.

Nero's Domus Aurea had occupied enormous sections of the urban centre — including the sector between the Palatine and the Esquiline, already densely inhabited. Vespasian had a clear political strategy: to restore to the Roman people what Nero had taken from them. The partial demolition of the Domus Aurea and the draining of its artificial lake freed the space for the new amphitheatre.

The message was explicit: where the tyrannical emperor had built his private palace, the good emperor would build for everyone. The original inscription — not preserved but reconstructed through sources — mentioned the construction ex manubiis: with the spoils of war, probably from the conquest of Jerusalem in 70 AD.

The Site and the Foundations

The Ground Above the Lake

The principal engineering problem of the site was the nature of the soil: the depression between the Palatine, Caelian and Esquiline hills was occupied by an artificial lake, with a bed of lacustrine sediment and water-saturated earth. Building on this type of ground the largest structure ever erected in Rome was an extraordinary challenge.

The solution adopted was one of the great engineering achievements of antiquity: a double elliptical ring foundation system, founded on more than 200 concrete piers driven deep enough to reach solid bedrock. The foundations average 12–13 metres in depth, and the external foundation wall reaches a thickness of 3 metres.

Roman concrete — opus incertum and opus reticulatum depending on the zone — was the material that made this system possible. Composed of lime (from the calcination of limestone), pozzolana (Lazio volcanic ash with exceptional hydraulic properties), and various aggregates, Roman concrete achieved strengths superior to many modern concretes, especially in damp environments.

The Dimensions of the Foundations

The foundation ellipse measures:

  • External major axis: 527 × 457 metres (total circumference of the foundation perimeter)
  • Area occupied by the foundation system: approximately 2.5 hectares
  • Estimated volume of concrete poured in the foundations alone: over 100,000 cubic metres

The Materials: A Catalogue of the Italian Territory

The construction of the Colosseum mobilised materials from every part of Lazio and central Italy. Petrographic analyses over the past decades have allowed the origin of each element to be reconstructed with reasonable precision:

The Travertine

The great majority of the external structure — the 76 arches on four levels, the pillars, the arcades — is built in Tiburtine travertine, quarried from the quarries at Tivoli (ancient Tibur), approximately 30 km from Rome. Travertine was probably the most logistically demanding material to manage: blocks weighing 2–4 tonnes each, transported overland and then by the Tiber.

The quantity of travertine used is estimated at approximately 100,000 tonnes. A specialised railway (wooden, animal-drawn) was probably built to link the site with the river port.

The Tufo

For the internal partition walls, dividers and secondary structures, lithic tuff (capellaccio and Aniene tufo) was used primarily, quarried from the Castelli Romani and Capitoline quarries. Tuff is lighter than travertine and more workable, ideal for non-load-bearing structures.

Roman Concrete

As mentioned, concrete (opus caementicium) was the principal structural material for foundations, vaults and wall cores. Lazio pozzolana — particularly that from the Alban Hills — gave the mixture its characteristic strength, even in the presence of water.

Brickwork

As the Flavian building campaign extended toward the end of the first century, brick (fired clay) became increasingly prevalent in the walls. Mixed masonry (opus mixtum) — brick with stone or travertine reinforcements — characterises the repairs and additions of the Antonine and Severan periods.

The Metals

Approximately 300 tonnes of iron — in cramps, pegs and tie-rods — were used to connect the travertine blocks and reinforce the joints. The holes left by the medieval removal of the iron cramps are still visible at thousands of points on the exterior of the Colosseum.

The Structural System: A Masterpiece of Optimisation

The Honeycomb Structure

Internally, the Colosseum is organised as a system of concentric and radial galleries — a honeycomb structure that distributes loads from the cavea to the foundation as efficiently as possible.

The annular corridors (four for each side of the ellipse) function both as load-bearing structure and circulation space. The radial walls — eighty in total, one for each ground-level arch — transfer the cavea loads to the foundation piers. The system is highly redundant: even if one or two walls were to fail, the overall structure would remain stable.

The Vaults

The Colosseum was probably the building with the greatest number of concrete vaults ever erected in antiquity. The three main types used:

  • Barrel vaults: in the annular corridors and radial passages
  • Cross vaults: at the nodes between annular and radial corridors
  • Annular vaults: saddle-shaped, for the curved internal corridors

Concrete vaults allowed large spans to be covered without the need for permanent timber centring — an enormous advantage in terms of cost and time.

The Vomitoria System

The 80 ground-level arches — called vomitoria — were the numbered entrances through which the public accessed different sectors of the cavea. The name is no accident: the internal corridor system was designed to "vomit" (expel) 50,000 spectators in under 15 minutes.

Each ground-floor arch led into a staircase giving direct access to the corresponding row of seats. The system was managed via numbered tokens (calcei) — a seating code ticketing system not unlike a modern stadium.

The Workforce: Slaves, Technicians and Craftsmen

The question "who built the Colosseum?" has a complex answer. Ancient sources preserve no detailed records of the workforce. Historical interpretations oscillate between two extremes:

The tradition of slave labour: the Roman conquest of Jerusalem brought tens of thousands of Jewish prisoners to Rome. Some ancient sources and many modern historians assume that the excavation, foundation and heavy material transport work was performed by this coerced workforce.

The technical complexity: many construction phases — travertine cutting, concrete vault laying, timber centring carpentry, geometrical survey of the ellipse — required specialised skills that cannot be improvised. These tasks were almost certainly entrusted to free professionals: fabri (masons/carpenters), lapidarii (stone workers), stucatores, and measurement technicians (agrimensores and gromatici).

An estimate of the total workforce varies between 40,000 and 100,000 people during the peak years (70–75 AD), with different tasks for different groups.

The Timeline: Eight Years from Start to Inauguration

Construction was completed in successive phases:

  • 70–72 AD: excavation, lake reclamation, foundation casting
  • 72–75 AD: construction of partition walls and corridors from the first to third levels
  • 75–79 AD: completion of the fourth level, installation of the marble cavea, installation of the hydraulic system for the naumachia
  • 79–80 AD: finishing works, decoration, inauguration under Titus (80 AD) — his father Vespasian died in 79 without seeing the work completed

80 AD, inauguration: 100 days of games. Sources report the death of 9,000 animals and an unspecified number of gladiators and condemned prisoners.

Domitian, Vespasian's second son, completed the fourth level and added the upper portico. The last documented construction finishing work dates from his reign.

Visiting the Construction Site of the Ancient World

The Colosseum you visit today is the result of one of the most extraordinary building sites of antiquity. Let your NCC driver take you there without stress — so all your energy goes to the monument. Service from €49. → Book your driver at myromedriver.com

Frequently Asked Questions

How long did it really take to build the Colosseum? Sources indicate approximately 8–10 years of active construction (70–80 AD), with some finishing work completed under Domitian after 80. For comparison, Cologne Cathedral required over 600 years; Barcelona's Sagrada Família is still under construction 140 years after it began.

Who paid for the construction of the Colosseum? Ancient sources indicate it was built ex manubiis — with the spoils of the Jewish War (66–70 AD). Vespasian and then Titus used the plunder from the destruction of Jerusalem — including the treasury of Solomon's Temple — to finance the project.

How many workers built the Colosseum? No direct sources exist. Modern estimates — based on comparative studies of Roman construction practice — indicate between 40,000 and 100,000 people at peak, with a mixed composition of slaves, freedmen and specialist free workers.

Why is the Colosseum elliptical rather than circular? The ellipse offers practical advantages: for the same volume, it provides a longer arena edge (and thus more spectators close to the action), and allows the entry and exit system to be organised more efficiently than a perfect circle. All major Roman amphitheatres are elliptical.

Are the holes in the external structure damage or original features? They are damage. The holes — visible in their thousands on the external façade — are the cavities left by the removal of the iron cramps that held the travertine blocks together. The metals were systematically extracted in the Middle Ages for re-smelting. An estimated 300 tonnes of iron were removed.

Article n. 19 — TIER S — MON-01 Colosseum Type: HISTORY Words: ~2,400

See also