I saw many buildings which seemed to have been built a long time ago. It felt as if I were in medieval times. I know that there are many students studying in those buildings. I imagine how pleasant it would be studying in those buildings every day as well. If I am accepted to this university then I would treasure my time there.

The other tutors have made the same suggestions that I would. However, I'd like to add that by writing "If I entered this university... " the use of "entered" is a bit odd. At least in the US, one would say, "If I attended this university..." Although you might hear someone say, "I entered the 9th grade in 2012," typically when you use "enter" with "university" it could mean that you entered a door into the university building (assuming there was just one building).

I saw a lot of buildings which seemed to be built A long time ago. I felt as if I were in THE medieval period. (A medieval period if there was more than one) If I entered this university, I could spend very precious time *doing something*. (...spend very precious time wandering the halls / spend time making precious memories / I could spend all day studying here / etc.) I hope this helps! :')

I saw a lot of buildings which seemed to be built long time ago. I felt as if I were in a medieval period.

I know that many university students are studying in these buildings. ("saw" ..." I know" ... "are studing". You have changed tense here to present. Was that intentional?, eg I know that many university students study in these buildings., or I knew.. which sounds a little strange.) I can imagine how pleasent it is to study in these old, beautiful buildings everyday.

If I entered this university, I could spend very precious time. "very precious time" is a little awkward. you could go with If I ... I would treasure time here." The time spent here would be precious. ("Precious" often has a sense of limited, as if one should not waste something with limited availability". So this may not be the word you are looking for. Not wrong, just some possibility of misleading.) . as a minor thing, I read "a medieval period" twice. It does not seem wrong. I wonder if there is one period called "the medieval period". so I'd either wite "the medieval period" or maybe "a medieval time" or "medieval times"