On only one side was it accessible by land.

This was the narrow neck of the peninsula, and here the several low hills were a natural obstacle.
Practically isolated from the rest of the world, the Fire People must have here lived and prospered for a long time.

In fact, I think it was their prosperity that was responsible for the subsequent migration that worked such calamity upon the Folk.

The Fire People must have increased in numbers until they pressed uncomfortably against the bounds of their habitat.

They were expanding, and in the course of their expanding they drove the Folk before them, and settled down themselves in the caves and occupied the territory that we had occupied.
But Lop-Ear and I little dreamed of all this when we found ourselves in the Fire People's stronghold.