One of Pasadena's most unique buildings, the Castle Green was built in 1898 as the annex for the famous Hotel Green. The Castle Green is an imposing seven story Moorish Colonial and Spanish style building sitting next to Central Park in Old Pasadena at Raymond and Green Street. The Castle Green was built by Col. George G. Green of the Patent Medicine Business.

The Castle Green, opened in 1899 as the second of three buildings in the Hotel Green complex, was a lavish resort for easterners and others escaping winter rigors. Architect Frederick I. Roehrig, for what was later to be called the Central Annex, drew on Moorish, Spanish, Victorian, and other stylistic elements to produce Pasadena's most stunningly original building. He blended domes, arches, pillars, balconies, and verandahs in a building of structural steel with brick walls and concrete floors, making it Pasadena's first fireproof building. He tied it to the first part of the hotel complex, built on the east side of the street, by an ornate enclosed bridge crossing Raymond Avenue. When the Annex opened for business, its two cylindrical towers on the south and much of the roof line were illuminated with exterior lights. There was even a penthouse garden with a glass roof.

The Hotel Green became the social center of Pasadena, playing host to vacationing tycoons and even presidents. It was also home to both the Tournament of Roses and the Valley Hunt Club.

At the end of the resort era, the west annex was purchased by a group of regular hotel guests who wished to continue to come to the Hotel Green. In 1924 the group of investors divided the hotel complex into three parts. The Central Annex was divided into fifty individually owned units and so it remains today, renamed the Castle Green.