The Puerta de Alcalá stands in the middle of Plaza de la Independencia like a massive stone bookmark marking where old Madrid ended and the countryside began. I've walked around it countless times, and it never fails to catch my eye – this neoclassical monument has a presence that's hard to ignore, even in a city packed with impressive architecture.

Francesco Sabatini designed this gateway in the 1770s, replacing an earlier, more modest entrance from 1599. What strikes me most is how different the two sides look. From the east, you see five openings – three large arches flanked by two rectangular doorways. Turn to the west side, and there are just three arches. It's like the monument has two personalities, which somehow feels very Madrid to me.

The thing about the Puerta de Alcalá is that it's completely isolated now. Unlike other city gates that connect to walls or buildings, this one sits alone in its roundabout, surrounded by traffic. Cars circle it constantly, making it feel both central and oddly detached. You can walk right up to it, but you're always aware of the urban chaos spinning around you.

The sculptural details are worth examining up close. Roberto Michel and Francisco Gutiérrez Arribas carved figures and reliefs that celebrate military victories and royal power – typical 18th-century propaganda in stone. The Spanish coat of arms crowns the whole structure, and there are cornucopias and various emblems scattered across the facade. It's the kind of monument that was meant to impress visitors arriving from the east, showing them they were entering an important capital.

Discover all the secrets of the Puerta de Alcalá with our complete Madrid audio guide and you'll understand how this gateway connected the royal court to the rest of Spain. The old road to Alcalá de Henares passed right through here, bringing news, goods, and people into the heart of the empire.

Standing near it on a quiet morning, before the traffic gets heavy, you can almost imagine what it felt like when this was actually the edge of the city. Now it's just another stop on the tourist trail, but it retains that sense of being a threshold – a place where one thing becomes another.