The ermine is actually an unusually beautiful NORTHERN weasel whose fur is brown in summer but white in winter.
Both are adjectives with different meaning .
Northern > located in the north / or LIVING in the north as in northern weasel/ northern invaders attacked France .. .
North > facing the north as in ' Go to the north gate'
So, northern gate and north gate have the same meaning .
Northern gate is the gate that is located in the north ( of your compass)
North gate is the gate that is facing north or towards the north (of your compass)
North Asia = Northern Asia .The choice of the better word is up to experience as to which is more commonly used
The north of your city is the northern part of your city.
Noun is also a noun > the direction it is coming from or to . He came from the north . Winds blowing to the north .