The stone traditionally used to build cities contributes to their personality and attests to the geological substrate on which they stand. While stone decay in the built heritage can be attributed to a number of causes, anthropic activity has a particularly significant impact. The geomonumental routes project is one of the initiatives proposed in recent years for urban routes that convey geological fundamentals by observing the rocks present in heritage structures. Its innovative approach addresses traditional stone properties, original quarrying sites and mechanisms of decay. Madrid's Royal Palace is a fine example of the use of traditional building stone in the centre of the Iberian Peninsula. In the geomonumental route proposed, the building doubles as an in situ laboratory that affords an overview of the main petrological properties of the two traditional stones most commonly used in the city's built heritage, the forms of decay they are subject and the factors underlying such alterations. This route constitutes a tool for showing the main petrological features and decay forms in traditional building stones found in urban heritage façades, with a special focus on anthropic impact, primarily air pollution and the use of conservation treatments that time has proven to be unsuitable.