Edit: This should be Post #12, after Tim's.
Then I wonder (as has Apollonius on the previous page) how Salazar Slytherin installed a secret chamber that you can access through one of the current castle's bathrooms...Also, wouldn't the very uncertainty of the date of Hogwarts' founding suggest that the current castle dates from after the Founder's era?
Gothic style is something that is very easily put on to existing buildings. If you have a look around some cities in Italy, for example, you'll se that a lot of them have had gargoyles and decorations in the marble attached. Something new came into fashion, the buildings didn't fit anymore, so they were altered rather than torn down and rebuilt. I can see the same thing happening with Hogwarts, especially since a bunch of Gargoyles would be so easy to attach when you have magic.