I will need help with this as it might be in a book!
It's in the southern center of the continent, surrounding by mountains with only one entrance, apart from an ancient secret entrance that's only accessible through a river filled with terrible, deadly, murderous monsters, and only one being has crossed the river: Xilon Killfury.
There are three small farming towns scattered around the plains inside the mountainous border. Well the population... I think I said was near 200k ish. As for the terrain, apart from the mountains surrounding it, there are only plain fields and one or two farming villages, small ones, but enough of them to support the city. The terrain of Rathengar used to be a deadly corrupted land, only one spot of that corrupted land remains. On the West side of it is thinned out forests, being very thin near the ocean border slightly getting thicker as you move east, but then starts to thin out again as you reach the start of the words of the first picture i gave you. As for Rathengar, there is a huge circular mountain "border" surrounding the city (with enough room for three farm towns, and quite a bit of open plains, though Rathengar itself should still be quite big.) there is one "secret" river in to the northern side of the mountains, this land is corrupted and is the only remaining corrupted land, the river is surrounded by forests and cannot be seen, it leads to a lake east of Rathengar, (not very east, a mile or two) The entrance and exit of the river should be hidden from view. The entrance to the mountainous border is south east. There is only one. As for the very north of the continent that Rathengar is on, the terrain is VERY mountain-y, but only on the very ocean border. The very east side is a rejuvenated desert, has the terrain of a desert but no sand, and what I mean by that is that he has lots of those sand mounds (but instead of sand it's now dirt and grass.)