The area where Bruma now is was originally Settled at the end of the Second Era by the soldiers of King Cuhlecain, who ordered the construction of a great fort/castle in the area that would consolidate his control over Pale Pass; serving as an impenetrable barrier with its strong garrison together with the Jerall mountains, in case any Nord or Reachmen tribes managed to push his forces out of the distant and difficult-to-govern Falkreath.
Unfortunately Cuhlecain died before it could be finished, but Tiber Septim embraced the project and completed it, making it the base for one of his legions. The advantage of having such a fortress is that if any southern threat were to capture the Imperial City, Tiber Septim and his armies could fall back to the very defensible Jeralls, where he would be backed by the armies of the Nords of Skyrim, who were fiercely loyal to him due to his alleged Atmoran backround and all that.
Later on during the Septim Dynasty rule, the fort remained an immensely important military asset, with a whole legion occupying it. Since this fort was in such an isolated, cold place, many Nibenese and Nordic people decided to settle the area surrounding the fort, looking to make a living by offering services to the legionaries. Like that, a sizable city grew around the fortress.
In 3E 110, Antiochus Septim ordered the extension of the walls to encompass the city due to his overestimating the threat of the Maormer who had begun an invasion of Summerset that year, so a proper defence was needed if it became necessary to fall back to Bruma. Observing the size and importance that Bruma had accumulated, and the political power vacuum in Northern Cyrodiil Antiochus decided to give Bruma the status of County seat. But this county would work diferently from other counties. The countship of Bruma would be non-hereditary. Instead, it would be associated to a military rank, namely, the general of the legion based at the fortress, and it is often granted to generals most favoured by the Emperor. This tradition still remains to this day.
The city would be patrolled by the soldiers of the legion, and have architecture that mixes Colovian and Nord influences (somewhat like Solitude in TES V, I suppose), while having traces of the colourful, exotic Nibenay. Economically, besides the services to the legionaries (brothels, taverns, bathhouses?) an industry has grown around the good quality timber found in the area. I picture large tree trunks being floated down a river towards the Imperial City.
I had more ideas, but I can't remember everything.
