When you reviewing and looking back to the past time where people always share tubs for bathing, you can see how the tub is very opened and inclusive. It can be enjoyed by so many people at the same time. Men and women are mixed into one unit. Especially in the beginning of 13th century until 18th century, people always share bathing tubs and they called it as spa.
At that time it is very common to take a bath together in large number of people. But not now, since nowadays, spa is identically with the very private thing in which men and women have different rooms and places to bath. In 13th century, you can see that the hot tub, just like in Rome, is very huge so that it covers hundred people in one period of bathing.
It makes sense if there are so many people gathered in it to take a bath. But spa recently is much different with that. Now, spa has its own place and the most important is done indoor. Spa in 13th century was done outdoor since the hottub is outside of building. This happens since people are growing in thinking so that the spa is designed and done differently from the past.