? about the Chamber entrance and intro

[whirlillusory]

No: HPFGUIDX 47775

I haven't found the answer to this question, so I figured SOMEONE 
around here could help me out, or at least point me in the right 
direction! :D

If the entrance of the Chamber of Secrets is "behind" the sinks how 
is it that Salazar Slytherin could have built this entrance?  If the 
founders of Hogwarts lived 1000 years ago, that puts their time a 
little before indoor plumbing! 

TIA! 

When I post for the first time I like give a brief intro so... if 
not interested, just ignore the following!

I'm 27 years old, and a science teacher in a small rural high 
school.  Love reading, hiking, skiing, Star Wars (big time), movies 
in general, and lots of types of music.  For the past couple of 
years I've been working on my Masters degree and thank goodness 
Harry Potter finally smacked me over the head a year ago!  People 
kept on telling me I had to read these books and I always said I 
didn't have time.  Last Christmas break I read them solid all the 
way through and Harry Potter became my much needed stress break over 
this last year.  Any time I need to back off from teaching or being 
a student I grab a Harry Potter book and take a little vacation! :D