Did castles have indoor bathrooms?

Yes, according to Cliff Claven:

"17th century castles have indoor bathrooms, and the plumbing system dumped directly into the surrounding moats."

http://www.iveyengineering.com/historical-events-plumbing-systems/

The term "bathroom" needs to be defined. Today, many people think a bathroom includes both a bathtub and a toilet. Centuries ago -- and even when well-heeled Romans connected to the Cloaca Maxima sewer system had hot running water in their homes -- people did not locate the toilet/latrine/privy in the same room as the bathtub. You didn't clean yourself in the same room as you evacuated. Some Europeans today are similarly fastidious.

So, yes, some medieval castles had "indoor bathrooms," meaning rooms in which people bathed; and they also had indoor "privies," which people used for the same reason that toilets have always been used. In the Middle Ages, these were not flush toilets, and the bathtubs had to be filled by hand, with the water being heated on coal or wood-fired ovens.

There could be a problem of interpretation here, as Frankie points out. To Brits, a bathroom is a room with a bath so the idea of a restaurant with bathrooms is somewhat surreal. When a bathroom and loo are combined, the name given to the room depends on which reason you are going to it. (My house - built in 1870 - was originally with no bathroom and with an outside loo as was normal practice at the time. It was ingeniously converted in the late 60s so that the end of the landing became a combined bathroom/loo.) I think zbeck's answer is using the American usage of bathroom to refer to garderobes, which were draughty holes in the wall for the purposes of evacuation (not getting away, that was via the postern). Bathing in those days was rare. Elizabeth I bathed regularly. Once a year. And some regarded that as excessive. Bathing in a northern (as opposed to Mediterranean) castle would have been a hazardous undertaking, bearing in mind the fearsome draughts and stone floors. Carpets were used, but they were hung on the walls to try to cut down the draught. And in medieval castles, there was no glass in the windows. When a knight was going for his vigil and knighting, he would take a bath - which indicates to me at least that this was not an everyday occurrence and was possibly part of the ordeal.

There is a castle standing that DOES have an indoor bathroom lol :) Its the Castle of Muskogee Oklahoma.

http://www.okcastle.com/