I think "over" should be accepted, too (as if the builiding is constructed like a bridge)
Reminds me of the great and spacious building which stood in the air, on the other side of the river of water.
I translated "... egy folyó" by "... one river" and got corrected. I thought since egy = one (the numeral), it could be used in this context. Is there a reason it's not possible here?
In Hungarian we make a definite difference between the numeral "egy" and the pronoun "egy" and it is clear from the context that the example is not about one river of the many but "a river".
If the building was above a river it would sink ! in english we would say it is built on the bank of the river
Not necessarily. London Bridge at least used to have buildings on it many years ago: mainly shops, I think. So too with Paris.