Gate of Erebor
From the southern slopes of the Lonely Mountain, two long ridges of rock were thrust out, forming the narrow valley in which the town of Dale stood. At the head of this valley was the wide arched entrance of the Kingdom under the Mountain, the Gate of Erebor. Out of the gateway sprang the stream that grew into the River Running.
The gate was the site of at least two sieges. In the year he reclaimed the Lonely Mountain, King Thorin needed to defend his ancient home against Elves and Men, and built a strong wall across the Gateway, though at that time battle was avoided. His successor Dáin II Ironfoot was less fortunate: he fell in battle during the War of the Ring at the threshold of the Gate, but it protected his people in the short and unsuccessful siege that followed.