• Brief description: The Royal Palace of Madrid is the official residence of His Majesty the King of Spain, which the King uses for ceremonies of state but does not live in. It is considered the largest royal palace in Western Europe in terms of length, with 135,000 m² and more than 3,418 rooms. It houses a valuable historic-artistic heritage, including the Stradivarius Palatino quintet, the most important collection of these instruments in the world. All the rooms that are open are worth visiting. 
•  Cost: 11€ entrance only. The audioguide costs 4€ and the guide 4€ (if you understand Spanish or English, there are little signs that explain everything about the palace).
• Length of visit: 1 hour.
• Hours: April to September, from 10:00am to 8:00pm. Rest of the year, from 10:00am to 6:00pm.
• How to get there: walking. It's in Plaza de Oriente (Metro Ópera L2 and L5).