Went there twice during my 2 weeks stay in Aachen. Had to plan a 2nd visit as I could not get a massage walk in appointment. Do book ahead of your visit to avoid disappointment!
Staff are friendly and helpful during my first ever visit. Towels and robes are on rent. Just need a robe and 1 towel - sufficient for pools and sauna. And yes, sauna is unisex and you will have to strip down. You can bring along a small bag and throw in your book, shower gel etc and bring it to the poolside.
Everything will be recorded on your chip - wristband while all your valuables are safely stored away. Had a nice lunch at the bistro as well, overlooking the main pool.
Absolutely loved the outdoor whirlpool!