A few thoughts on both of these:

In general, I think Spa Castle is superior. The men’s nude area in particular is larger, more updated, and the pools and sauna/steam room are much larger. I love this facility. It’s maybe a little less traditional, but more luxurious. Many of the pools have lounges in them which is lovely, and powerful jets to massage you.

King Spa seems a little more traditional and less fancy, but it’s still very nice. One major issue is that the steam room is always WAY too hot, like it’s unbearable. Hardly anybody goes in there, I honestly think you could get a scald burn. The pools are smaller and don’t have the lounges or jet features that Spa Castle has.

The other non-nude areas of both are nice, and fairly comparable. Spa Castle has bigger pools.

Happy to answer questions here or through chat! I’ve been to both of these many times.