Madrid

Unofficially, it became the capital of Spain in 1561 when Philip II moved his court here. This move accelerated the city's development.

An excerpt from the article 17 facts about Madrid
Although in the middle of the 16th century it had a population of just over 20,000, by the end of the century the population was close to 100,000. A few years later, it was decided to move the capital to Valladolid, which meant that in 5 years the city was deserted again - up to 60,000 people left it.