The Forbidden City is a palace complex located in the center of Beijing. It was home to the Emperors of both the Ming and Ching dynasties (along with their hundreds of servants and eunuchs). The city was completed in 1420 (Ming Dynasty) and was occupied continually until 1924 (when it became a museum). It is protected by high walls and is completely surrounded by a moat.
The Forbidden city has also been called the purple city, but that is a mythological reference to the North Star and its symbolic connection to the Emperor (and has nothing to do with the color purple).
Normal people could not enter the Forbidden City. Even high ranking officials were rarely allowed inside (and always with good reason when they were). So for centuries the city was a place of mystery, a place where common men could not go. Hence the name, "The Forbidden City".
Places inside the complex include the Gate of Supreme Harmony, the Hall of Celestial Purity, the Hall of Preserved Elegance, and the Pavilion of Cheerful Melodies. Overall the complex could house thousands, and even today it is a home to many treasures.