Can't

Clerebold immediately understood the conclusion those 12 Clans would come to. Maybe there were already armies marching toward them currently. They didn't have much time to prepare at all. 

Though there were natural barriers between the regions, one needs to remember that there was a ring of nothing but flat plains around the core region of the Ancient Battlefield. Dyon had run across this very flat plain when he entered the core region for what remained of the Pride Clan's Legacy. 

What did this mean? It meant that even if Clans couldn't take a straight line approach to reach their territory, what they could do was take a detour to these flat plains. Using this defenseless region, they could wrap around the natural barriers and enter the east unhindered. This meant that the dwarves wouldn't only have to deal with attacks from the eastern region, but may also have to face attacks from other regions as well.