This programme looks into the structural foundations on which all buildings rely in order to keep them upright. We explore the ancient leaning tower of Pisa, the incredible Puerta de Europa towers in Madrid, and a modern classic, the Torre de Collserola in Barcelona. How Did They Build That? is a series that explores how new materials and construction techniques have allowed engineers, builders and architects to build some of the greatest man-made structures of all time. Published: 13 February 2008