This question already has an answer here:
Hogwarts Castle was built ~1000AD by Salazar Slytherin and the other three founders. But indoor plumbing (i.e., running water through pipes) wasn't developed until - well, I'm not sure, but apparently it wasn't until the 1800s. Now, Pottermore confirms it: https://twitter.com/pottermore/status/1081242428105998336
So the secret entrance to the Chamber of Secrets clearly wasn't via some kind of sink outlet, ornate or not.
And I believe it unlikely to have been through the medieval equivalent because, though it certainly would have been an excellent disguise for it, I can't see Salarar Slytherin building it that way for reasons of dignity.
So, what was the original design, when was the Castle renovated with indoor plumbing, who built the current secret entrance during those renovations, and how was it kept secret from all the other occupants of the Castle at that time and also the other renovators?