Japan has some natural barriers against invasions. Firstly, it is an island, rendering a land invasion impossible and making retreat harder. Due to geographic conditions, Japan's lands have strategic points of advantage for Japanese soldiers. The country is mountainous and full of hills, mostly product of volcanic eruptions, presenting difficulty for invasions, especially once locals now how the best routes and positions. Due to these conditions, Japan has some regions more viable for invasion, leading the country to gather their best defenses at these spots.
