While not exactly a castle, it was certainly a well-defended home...
...perched amid the upper branches of one of the prickliest of plants, a well-grown cholla cactus armed with long thorns and, less visible, the tiny, vicious glochids that characterize the Opuntia species.
I'm not sure what kind of bird built the nest, which is covered over the top and has only a small round entrance.
It did not appear to be in use just now in early August, but it had certainly been a fine home, built with care.
Thanks to my sister for discovering this dwelling down in a nearby arroyo. See also my post at the Small Sunny Garden for a more complete look at the desert on a dry August morning.