Now I'm truly confused.... I was jumped by a wither skeleton in the nether near our base camp... that's two mobs which are reported to exist only within nether castles... but I've dug high and low around our base camp area... no castle anywhere.
Either way, I've begun an expedition in the nether for another castle closer to home... no luck so far. How DC found that first one amazes me given how far away it is.