This was the second time this year we have been in Hotel Balneario de Mondariz. Our stay last September was more pleasant than December one, because of the weather which able us to get sun and use exterior swiming pool (free of charge for guests) but also because Palacio de Agua (Water Palace) facilities (SPA) was not full.
I guess this weekend (05 to 08 December) and because of bank holidays, the hotel was full so waiting for a while to get a table for breakfast. Breakfast is good but maybe not up to a standard 4 start level hotel.
Rooms are big, corfortable, clean with small windows, sleeping very well and calm.
Palacio de Agua is the name for the SPA, with many water jets, jacuzzis, bubbles beds, etc as well humid and dry saunas, where you can stay and use for a maximum of 3 hours, this of course if you book the room with SPA otherwise you must pay around EUR 25,00.
They have too a Celtic Bath but I did not try.
The surroundings are beautiful and you can walk in the forest following some tracks or join the ones the hotel promotes.
Hotel restaurant is very expensive, about EUR 39,00 per person, so we went to nearby restaurant were you could eat well about half of the price.
There is a bar too where, at night, you can enjoy a piano and violin session while enjoying a drink.
If I repeat the experience, yes with good weather and profiting of promotions like "Valores Anadidos" the hotel does sometimes, otherwise is expensive.

We went to Mondariz in a week-end and we stayed in Gran Hotel. The Hotel is not a 4 stars, it has some minor details that co promise the 4*: shower curtain, bad insonorization, old decoration, but at least the room had a lot of space. The Palacio del Aqua is like a massive destruction SPA. Loads of children screaming, too much people for the small space available. The pool is an interesting concept, but with less 50% of occupation... The outside warm pool would be very nice with less customers. The only fine hour to visit is from 20:00 until 01:00. Yes it is open until that hour and is the only possible period to relax and enjoy...