The map you linked doesn't show it, but there are mountains to the North and East, which probably gave it a bit of protection. But yeah, the Romans only managed to hold on to it for 175 years, which is pretty short compared to the rest of the empire...
The quirks of .svg maps/links I guess, maybe this one works [1]
As per the mountains, they are not that impenetrable, but you're of course correct, the easiest way was to come through Porolissum, through the North-West passage, the "mountains" there do not surpass 700-800 meters in height.
That's how the Magyars actually did it about 800 years after the Romans, i.e. they came through today's county of Salaj, where the old Porolissum used to be. If I'm not mistaken the name of Turda itself comes after a Magyar ruler/chieftain called Torda (a Turkic-like name itself, but that's another discussion).
