I combined different pictures of the walls from different sides of the castle.
In short: I took a photo from each side of the the castle (which is rectangular).
I did modify one of the wall photos though, in GIMP, because of the light coming from that side.
(ie. desaturate it to balance the color with the other wall. I also used the Clone Tool to get rid of other stuff such as grass in front of it.
I’m thinking about using GIMP to combine them into one gigantic wall-texture to be used elsewhere. It wouldn’t be seamless though, but neither does it have to, unless you are making a gigantic wall model too, without any windows or other stuff.