One is Blood Hill Mountain Range which is in the middle of all three Empires. But the other great forest, Magical Forest is towards the west of the Continent and divided the whole continent into two parts.
The Eastern Side of the Continent houses three Empires and on the Western side Two Alliances. This Magical forest is the reason why the Three Empires couldn't invade them easily and acquire their land. But the three Empires are still too powerful, And if the power on this side remained scattered then Empires could easily dominate them. That is the reason for Two Alliances. Even though the land the two alliances have is almost half of the three Empires, they were still able to remain in power.