The former site of the Banco de Bilbao in the Azca financial district in Madrid is an expression of the organic conversion of architecture towards the features of modernity.
This is one of the most important works by the architect Sáinz de Oiza due to its dimensions and location, and also because of the engineering challenges involved in building directly over a railway tunnel.
The tower has a rectangular ground plan and a height of 107 m, and was based on ideas of an organic nature: a central structure integrating the various parts of the building divided into platform-blocks (the floors) of different heights.
However the tower's exterior language is purist and simple, not so much an image of rationalism, but merely an analogy of the natural way the skin covers an organism. The façade is formed by metal shades and rounded corners in ochre tones as a result of the gradual rusting of the steel.