Sacsayhuamán: how the Inca actually fitted those stones

Sacsayhuamán stands above Cusco: three terraced walls of limestone, zigzagging for several hundred metres, built from irregular polygonal blocks fitted so tightly that a knife blade will not go between them. The largest weigh well over a hundred tonnes.

It is the standard exhibit for the claim that ancient people could not have done this. It is also one of the better-documented construction projects of the pre-Columbian Americas.

What is documented

The Inca built it, in the fifteenth century, and the Spanish arrived while people who had worked on it were still alive.

Spanish chroniclers — Cieza de León and Garcilaso de la Vega among them — recorded accounts of the construction from Inca sources: the size of the labour force, the use of the mit'a system of rotational obligatory service, ramps, ropes and levers, and the deaths that occurred moving the stones.

Garcilaso, born in Cusco to an Inca mother, described a block that killed thousands of men when it got away from them on a slope. That is a construction accident recorded by someone with access to people who remembered it.

How the joints were made

This is the part that looks impossible and is the best understood.

The blocks are not cut to a plan and then assembled. Each one is shaped to fit the specific stones already in place. A block is lowered, the contact points bruise the stone below, it is lifted, the high spots are dressed away, and it is lowered again. Repeat until it seats.

That is why the joints are irregular polygons rather than rectangles: each face is a record of the stone it met. It is enormously laborious and requires no tool more advanced than a harder hammerstone.

Experimental archaeology has reproduced the technique. It is slow, and slowness was not a constraint for a state that could compel labour.

Why polygonal at all

Earthquakes. Cusco sits in a seismically active region, and interlocking irregular blocks laid without mortar can shift and settle back rather than shear.

Spanish colonial buildings in Cusco have been damaged repeatedly by earthquakes. The Inca walls they were built on top of are still standing. That is not a mystery — it is a design decision that has been tested for five centuries.

What the mit'a system actually was

The labour that built Sacsayhuamán was not slavery and it was not wage work. The Inca state operated a system of rotational obligation called mit'a, under which communities owed the state a period of labour rather than a share of produce.

A community sent workers for a defined term, the state fed and housed them while they served, and the obligation then passed to another community. It is how roads, terraces, storehouses and fortifications were built across an empire that had no money, no wheeled transport and no writing in the usual sense.

That system is the actual answer to how the stones were moved. Not a technique — an institution capable of concentrating tens of thousands of people on one task for as long as it took.

What was destroyed

Most of it. What stands is the foundation courses of a much larger complex, because the Spanish quarried Sacsayhuamán for two centuries to build colonial Cusco.

The blocks that remain are the ones too large to move. That is why the site looks like it was built entirely from giants: the manageable stones were carted away, leaving a selection biased entirely towards the enormous.