What they saw
From far, the spiral sculpture looks like a very long cuboid twisted to form the sculpture. However upon closer inspection, we realise that the sculpture s actually made up square blocks stacked up together. Below is a view of how the sculpture looks like closed up.

How it is made
It is made up of 100 pieces of white marble. Each measuring 17.0 x 140.0 x 140.0 cm. The basic idea of how the sculpture is made, is to rotate the next square block and to stack it onto the former one.An example is shown below

the subsequent block is rotated and stacked onto the former one
Finding the angle
By drawing a line from the midpoint of one side to the centre and doing the same thing to another square block using the nearest side, the lines drawn can be used to measure the angle of the rotation. as shown in the diagram below

a is the angle of rotation

However, for this instance, it is impossible to measure the angle using that way, as the sculpture is too tall. However, by visual inspection, we can see that the top square is at the same angle as the base, thus meaning the the squares have travelled 360 degrees, a full circle. As mentioned before, there are 100 blocks o square marble. So the angle of rotation for each individual square is 3.6 degrees (360 degrees divided by 100).