A tricky structure indeed. I concur with Casiopea's reasoning, but the sentence is still flawed (even with 'ago' replacing 'age'. It should be something like
"Beihai Park, built about three hundred years ago, is one of the most beautiful parks in Beijing."
or (if Francois has indeed grasped the intended meaning)
"Beihai Park is one of the most beautiful parks of those which were built about 300 years ago in Beijing."

In fact, now I come to think of it, I'm not entirely convinced that one can 'build' a park....