I love to look at real estate that I can't afford and imagine what I would purchase if I had the money for a castle! While Zillow.com and Realtor.com are common real estate perusing websites in the USA for normal websites, for castles, chateaus, manor homes, and palaces, I like to use the websites below:
CASTLEIST
Castleist is focused on castles of Europe but contains global listing. It's heavy on ads and often only has 1 picture per listing, but it's a great aggregate of many different real estate listings.
LIONARD
Lionard is a luxury Italian real estate company. As the listing agent for the real estate, they have many photos and it's a wonderful way to look at Lake Como villas, Venice Renaissance palaces, and medieval castles in Tuscany.
PRIVATE ISLANDS ONLINE
Private Islands Online is just a castle real estate website, their focus is obviously islands, but some of these islands have castles! The photos range in quality as often the Gilded Age style castles in the Caribbean are in disrepair.
LE FIGARO PROPERTIES
Website: https://properties.lefigaro.com/announces/chateau-real+estate-properties+for+sale-france/?tri=3
Le Figaro Properties is a French real estate company so their listings range from mansions to castles to chateaus.
DANIEL FEUA
This French real estate listing puts their map front and center, which is perfect for window shopping online!
COUNTRY LIFE
This British magazine features all types of property, from mobile homes to country homes. They also do round-ups of properties for sale, like this Scottish Castle list.
PRESTIGE PROPERTY GROUP
Website: https://www.prestigeproperty.co.uk
This UK real estate company has partners in over 8 countries so has a wide variety of locations.
SOTHEBY'S INTERNATIONAL REALTY
Website: https://www.sothebysrealty.com
The famous auction house, Sotheby's, has a wide range of luxury real estate and make it difficult to search for "castle" but is a great way to browse pretty homes!
Did I miss any luxury real estate websites you visit to look at castles?