Even on a tight schedule, CentroCentro / Cibeles Palace Rooftop earns the hours.
Allow 30–60 minutes.
A cheap, central panorama: the mirador gives sweeping views over the Cibeles fountain, Alcalá and the rooftops, and the building itself is a stunner. The catch is it's a small terrace with limited timed slots, and you queue for what's ultimately a brief view.