There were indeed breaches in different areas of the territory, but there were very few—embarrassingly few.
For the most part, the enemies were kept outside. This was especially true when the people 'blocking the way' were Alterra's elite forces.
For example, there was Oslo and his earth walls. Using his earth wall, he'd create small mazes and blockages for the humans, keeping them with the monster mobs for a longer period of time.
They would be forced to fight for their lives, but then the last kill would be stolen by a random Alterran archer, making their eventual death feel even more meaningless.
(Alterran archers were definitely enjoying their assignments right now)
Of course, there were plenty of stronger monsters that their archers were not able to take down. In that case, after the monsters ate a few members of the enemy forces, Oslo would then create spikes that'd take their lives.