Urban centrality is one of these complex concepts which are easier to recognize in everyday life than to describe with rigor and precision; this complicates (but does not prevent) actions to foster its positive effects.
As some have said, the center is, simply, this place to which everyone wants to go; to some this means, ideally, the place to which young people go to learn, but also to celebrate, and older people go to work, but also to enjoy, and there is also a lot of residents. To others, it is place with many roles, but few residents so they do not have to endure the burdens of congestion. And in real life, it is not so much a geographically defined space, with a closed shape, immune to change over time, but an area that evolves even along a single day, in which the most central areas are those that around the clock maintain for longer that centrality condition, always marked by the ability to attract external people.
What would be central Madrid? As it is a city with some history (although not that long, as somehow Madrid was to the European XVIth century what Brasilia has been to the American XXth), there are some physical limits; time and the inertia of capital infrastructure investments over decades have configured a space, coincident with the Centro ward, where converge the main transit lines and streets. It is the most iconic center nowadays, but up to the end of the XXth century it was the whole of the city, and this complexity is still apparent. The XIXth century Ensanche and the industrial extension towards Arganzuela only helped to stretch that center, an operation fostered also by the Gran Vía opening.
The consolidation of the Ensanche for a century and its better socioeconomic conditions as compared to the slums of Tetuan and Puente de Vallecas, almost contemporary in their development, helped the rise to centrality for that space, and more clearly around its western edge. And the northbound extension configures an axis around the Paseo de la Castellana in which by the mid 2000s there were already 3 jobs for each registered resident.
When we were called to write the Proyecto Madrid Centro (PMC), the City of Madrid considered that the scope for study and intervention should essentially be what is inside the M-30 beltway, which is as saying that the center of Metro New York is the whole of Mannhattan island, Paris 75 is the center of the Parisian metropolitan area, and the Ciudad Autonoma is the core of Buenos Aires. Work showed that this is just a simplification; on such scales the metropolitan dynamics and the fluidity of the transportation systems make for a more complex reality. Real centrality, as a combination of multiple roles able to attract external populations on a large scale, is the overlay of the Castellana Axis, the Centro ward and the Barrio de Salamanca, but there are also more local centralities; their future is threatened by the economic crisis, which seems to reinforce the attraction of the most central areas, aided by tourism flows, when compared to neighborhood centralities, more stressed in their retail role.