The views from the summit of the Palacio de Comunicaciones are arguably Madrid's best, sweeping west down over the Plaza de la Cibeles, up the hill towards the sublime Edificio Metrópolis and out to the mountains. But the views are splendid whichever way you look. Take the lift up to the 6th floor, from where the gates are opened every half hour. From there you can either take another lift or climb the stairs up to the 8th floor.
