Abaton was never/could never be taken.
A man (probably on the run from the law) went due west from the inner kingdom's largest city until he reached the mountains. They had long before been made impassible when the Old King reshaped all the lands to best serve His people, but the area met his needs at the moment because it was as far from other people as one could go without venturing into the southern desert.
One day, he found a healthy stream coming out of a small cave. Curious, he explored and found it wound upwards until opening onto a deep, richly forested valley high in the mountains. At the other end of the valley, the stream entered through another cave, one that led to the other side of the mountains!
Merchants and other travelers had to journey more than a week northward from the city he'd left so they could cross over one of the few mountain passes the Old King had left, and then they had to travel almost as far south to reach the best port on the coast.
Using his caves and passing through his valley saved so much time and avoided so many dangers, people were willing to pay high tolls. The man built an inn to extract a little more money from the merchants, a blacksmith paid for permission to put a shop next to it, and soon merchants wanted houses there so they could trade with the pack trains passing through. Artisans and craftsmen flocked to ply their arts there without being under the thumb of their guild or having to pay high taxes.
Stone taken from the caves to widen and level them went to building roads and walls. Trees felled for building houses left room for farmers to work the rich soil. The stream running through the valley was sufficient to power several mills.
Abaton quickly grew into the richest city in the kingdom. It's said there are fountains made of gold, and the Prince (as the man now called himself) found amusement in using jewels as large as hogsheads for doorstops.
She was easily defended. The gates to close the caves were not only strong, they had deep spells laid on them to rob the magic from any wizard who attacked them. The mountains on either side had been made so sheer, by no less a power than the Old King himself, no one dared to climb them. Her farms and fields supplied more than enough for everyone.
To ensure safety beyond imagination, a demon was crafted from the roots of the mountain and placed in a high tower to watch over city and protect her from all possible harm.