Trevor Johnson of Historic UK, an online accommodations directory and history magazine, says castle rates generally start around $88 per person but can exceed $1,755 a night for a luxury suite. For $3,510 and up per night, he notes, some places let you be ''king of the castle'' or ''lord of the manor.''

The cheapest castle bed in Britain ($22 a night) is at St. Briavel's (011- 44-1594-530272, http://www.yha.org.uk/hostel/hostelpages /233.html) in Gloucestershire, a moated Norman edifice with a dungeon. Originally a 13th-century hunting lodge, it's owned by the Youth Hostel Association of England and Wales. Good luck getting a bed: Manager Michelle Redhead says to book a year ahead. Open March-October.

The most expensive bed? Perhaps Skibo Castle (011-44-1-862-894- 600, http://www.carnegie club.co.uk), Andrew Carnegie's place on 7,500 acres near Dornoch, Scotland, reportedly the most luxurious private-member club in the world. Guests can visit once ($1,281 to $1,746 a night per person) but to return must apply for membership ($17,554 upfront, $7,021 annually). Besides meals, the rate includes golf, clay pigeon and target pistol shooting, cycling, tennis and other activities.

Sources:

l Historic UK (011-44-1788- 522341, http://www.historic-uk.com) is a booking site for a range of historic accommodations, including one in Northern Ireland. It includes photos of 71 castles for two to 100 people and choices for self-catering, bed-and-breakfast and hotel rooms, from basic to luxury. It's a good place to search for specifics and amenities. Prices are usually clearly stated.

l The Landmark Trust (011-44- 1628-825925, http://www.landmarktrust.org.uk) is a charity that restores old buildings and rents them to self-catering vacationers. Seven of its 180 listings are castles; some (Swarkestone Pavillion, Derbyshire) are gatehouses. The availability list is updated daily, and last-minute bookings are possible. Downside: You have to call for most rates. Order a copy of all properties for $25 from the Landmark Trust USA in Vermont (802- 254-6868).

l The booking company Celtic Castles (011-44-870-050-3232, http://www.celticcastles. com) lists 45 U.K. castles. The Web site features clear descriptions plus hot finds, such as a small private castle at under $100 per person; a currency calculator; links to castle Web sites; and child-friendly places. Some castles are owner-occupied and not hotels; the exclusive use of 14-bedroom Augill Castle in Cumbria, England, is $13,777 for two nights, while Amhuinnsuidhe on Isle of Harris, Scotland, costs $43,450 per week for up to 20 people, or $17,662 for two nights.

Scottish castles range from rustic one-bath Castle Levan near Glasgow ($105 per person for two to four people) to Stobo near Edinburgh, a sanctuary where $209 and up buys spa and exercise programs.

l Castles of Northern Ireland (http://www.discovernorthern ireland.com; search ''Places to Stay'' for premises with ''castle'' in their name) can take you to about a half-dozen possibilities, scattered among other lodgings that are castles in name only.

l Castles of the World (http://www.castles.org/Chatelaine) lists only Scottish structures but makes it easy to search by region and whether hotel accommodations or self-catering rentals are available.

l The National Trust (011-44- 870-4584422, http://www.nationaltrustcottages.co.uk), a charity that protects land and preserves buildings, has a Cornwall castle ($581 a week) among more than 320 places.

Packages:

Go-Today.com (800- 227-3235, http://www.go-today.com), a division of Europe Express, and Britain-based Virgin Vacations (888- 937-8474, http://www.virgin-vacations. com) are among the many companies that offer castle trips from the Washington area.

General information:

Check out Castles-of-Britain.com (http://www.castles-of-britain.com) for info about castle history and culture (plus lists with descriptions and links to their Web sites). VisitBritain, 800-462-2748, http://www.visitbritain.com. Wales Tourist Board, 011-8708-300-306, http://www.visitwales.com. VisitScotland, 011-845-22-55-121, http://www.visitscotland.com. Northern Ireland Tourist Board, 011-44-28-9023-1221,

http://www.discover northern ireland.com.