Click here for a page of figures and plans

In order to plan an amphitheatre
it is necessary to coordinate the dimensions of the arena, of the cavea (the auditorium), and the length of the external perimeter, which in turn is necessary in order to calculate the number and width of the external arches and the area occupied by the building. So, the first planning step established the dimensions of the arena, given the traditional ratio of 5:3 used for most amphitheatres.  Also, the width of the cavea had to be equal to the width of the arena, and in the case of the Colosseum this figure is roughly equal to the height of the external facade. Here are more details about the geometry .

Small differences in construction details have convinced the archaeologists that there were four different contractors, each one building a quadrant of the Colosseum. They would have worked side by side sharing the four main entrances. 
The name of the architect, like that of many others of the antiquity, is not known.
The money necessary to finance the building came most probably from the booty of the Palestinian war and the plundering of the Temple of Jerusalem.
This was widely accepted before as commonsense, but recently this theory has been corroborated by a recent discovery on a marble inscription. On the stone, “underneath” the inscription, there are still the holes used to lodge the metal letters of a precedent inscription, that was later erased.
The holes  were recently interpreted by Prof. Geza Alfoldy of Heidelberg University, who, working with Italian archaeologists, deciphered the puzzle.
He concluded that the first inscription read: “Imp. T. Caes. Vespasianus Aug. Amphitheatrum Novum Ex Manubis Fieri Iussit”, that means “Emperor Caesar Vespasian Augustus had this new amphitheatre erected with the spoils of war”. 
See here the original paper in German and here an explanation in English.

Little is known of the expense, which must have been must have been enormous: in 1756 the French mathematician and physicist François Jacquier calculated the cost of building only the external wall. Converted to modern Euros, keeping account of devaluation etc., the cost would be today of more than 40 million EUR.

Scholars have debated at length if forced labour had been employed to build the amphitheatre. In fact, slave labour was widely used then, however the concept that only slaves were employed has been rejected, since the quality of the construction is evidence of a skilled work force.

Click here for an explanation of the building strategy, according to Cozzo.

Once completed, the foundation base was covered by a travertine floor, 90 cm thick in average. On this stone floor were marked the reference points for the main pillars, and the base blocks of the pillars were anchored to the floor by a pivot and melted metal. 

This skeleton of pillars was raised up to the second floor, and the pillars were connected, at the top, by big arches made with 2 feet long bricks, placed so as to allow the construction of many rampant vaults, which all together make up the big cavea, destined to support the marble seats. Some remaining vaults can be seen in the picture of the arena as it is today.

The system of having a first basic structure built up to the second floor allowed the builders to carry out the rest of the works above and below the cavea at the same time, leaving only some vaults open for the lifting up of the materials. The space between the pillars was filled by tuff opus quadratum on the ground floor, and by cement aggregate with a brick facing for the second floor. The tuff structures and the bricks ones which constitute – together with the pillars – the radial walls of the amphitheatre are indeed independent from the pillars themselves and from the big vaults, and it is thought that they were built after the pillars.

In general, the different materials that were used (travertine,  tuff, brickwork and cement) were utilised by exploiting to the full their respective qualities of lightness, resistance and ease of installation. The combination of different materials has also improved the resilience of the whole structure.

Click here for an interesting study about the structural behaviour of the Colosseum over the centuries.

There are many outstanding structural elements in the Roman Colosseum: the formal elegance, the solidity of the construction, the organisation of the spaces (the entrance/exit system, the underground rooms which were like the backstage of a theatre). In my opinion one of the most astonishing feats is the building technique, i.e. the system of making first the main arches in travertine, so that the rest of the construction could be carried out at the same time above and below this first structure.

Amazingly, it took less than 10 years to build it! Ingegner Giuseppe Cozzo, who in the 30s excavated and cleaned the whole building, wrote (the translation is mine): “This building procedure, both simple and evident, allowed to terminate very rapidly the construction of the big cavea and to cover the walkways on the second floor; at the same time it allowed for the coexistence of two vast building yards on the same site; a first one, below, completely covered, sheltered from the rain, and a second upper one, above the cavea, in order to build the upper part of the amphitheatre. Below, under coverage, all the walls between the travertine pillars, the rampant bridging of the stairs, the vaults of the ground floor walkways, the plastering and the stuccoes could be completed; above, work could be carried out on the walls of the remaining two floors of the amphitheatre, the podium, the seats, and the wooden upper portico“.

The travertine blocks were all connected to each other by iron clamps, which have been extracted in medieval times, and have left the holes that can be seen everywhere. It has been calculated that – only for these clamps – the builders used 300 tons of metal.

The inside of the external travertine wall (see the photo) shows that many travertine blocks were recycled from other buildings: their internal face is in fact irregular, and they have been leveled only on the external and contact sides. We are not sure if the wall was constructed in this way so as to save time or if these irregularities depend on later restorations.

The external wall was once coupled and supported by a thick brick facing, and it is another mystery of the Colosseum how the wall you see can still stand. Some answers to the questions about the solidity of the monument can be found in this interesting paper on the structural behaviour of the Colosseum during the centuries.

Most of the information on this page come from G. Cozzo, Il Colosseo, Roma, Palombi, 1971;

Another interesting source is The Colosseum: Quality and efficiency of construction, by Giovanni Manieri Elia (with an English translation)