The terrain of Shadow Valley is extremely peculiar.
It is separated from the outside world by three ravines, which means large-scale military movements are nearly impossible unless descending from the sky or tunneling underground.
It's safe to say that it was Avalon's blessing that the shadow half-elves managed to find this place. They hid in this valley for over sixty years, disturbed only once by a gnoll tunnel raid, but otherwise left untouched by the outside world.
On the flip side, such a challenging terrain makes it difficult for the defending army to launch large-scale offensives, which is why Field Marshal Hass declared that the Transia people stationed here couldn't achieve much.
The Jackalman General wasn't speaking without basis.
