That's actually very good. Very proper English, my friend.

The route through the Nisava valley has been a major gateway to Europe from Asia since ancient time .The route pass through the valleys of Morava and Marista before it reaches Istanbul . The Belgrade- Sofia- Istanbul Highway and a railway line also follow the same route. The Nisava valley , which sits in between Asia and Europe , is a major natural gateway to to Europe from Asia , and then it passes through the valleys of Morave and Maritsa before it reaches Istanbul

Not quite. Your '...and onwards...' would be better as '...rivers, and continues onwards...' ': the' should be '. The'

Thank you very much :)

Is this correct? (not It's ...) The English is good. Punctuation could be improved. Maybe "that, from ancient times, ", and "Asia. The" A semi-colon would be better than a colon there and we do not leave a space before a colon or semi-colon.