The Éothéod, the ancestors of the people of the Mark,  were a nomadic people who roamed the wide northern lands, following the seasons in search of good grazing for their vast herds of horses. 

With the granting of The Gift of Allegiance (the ceding of the province of Calenardhon by Cirion, steward of Gondor, to Eorl and his people) the Éothéod found themselves for the first time in a secure country of their own - a country bounded by clearly demarcated borders and one in which settlements and strongholds, such as the Hornburg and Isengard, already existed.

The gift of the Mark was not one given without recompense, for this denuded province of Gondor was of vital strategic importance in the constant strife with that Kingdom's enemies in the West. The Eorlingas now became a shield against incursions and attacks by the Wildmen of Dunland, and the vast armies of the goblins and orcs who held sway in the southern reaches of the Misty Mountains.

And so circumstance and expediency led the sons of Eorl to establish permanent settlements and strongholds of their own, though many of the Rohirrim, eschewing the strictures of regimented 
governance and walled cities, continued to roam at will across the Mark's wide, unfettered plains.
The first new settlements appeared at Alburg, Eorl's hold, and Edoras. Both of these cities grew rapidly with accommodation and commercial buildings springing up within their walls. Farms and homesteads spread swiftly over the fertile country which surrounded these cities. Several villages sprang up along the vale of Harrowdale which ran along the course of the Snowbourne River to its source in the White Mountains. 

A great number of folk settled in the green vale known as the Coomb, situated in the Westfold near the great fortress of the Hornburg.  A series of ranches and villages occurred around the stronghold at Isengard and along the banks of the rivers that wound their way across Rohan's broad plains. Villages, homesteads and farms became abundant along the Mering Stream, Rohan's eastern border with Gondor.

The areas south of the eaves of Fangorn were scarcely populated, for the Rohirrim were a superstitious and suspicious folk and were loath to dwell to close to the dark, mysterious shadow of this great wood. To the east of the Entwash Vale the long grassy plains gave way to lesser grazing and the tall, inhospitable, hills of the Downs and Wold. Here of old were found a few small settlements. But, for the most part, the area was home to wandering clans, who set up temporary camps for some months until the seasons changed and they moved on in search of richer pastures.

Down the centuries the people of the Mark, and their horses, burgeoned and prospered. With the conquest of the goblin tribes and Wildmen the Riddermark's border pushed ever westward, so that during Aldor's reign large ranches and settlements sprang up in the lands between the Adorn and Isen Rivers (The West-march.)

At the time of the War of the Ring the country's western borders had diminished due to encroachment by Wildmen and Uruks at Saruman's behest. The eastern borders along the Anduin were subject to similar strife as orcs of Mordor carried out raids with virtual impunity.

A visitor to Rohan during the latter part of Théoden's reign would have found the border territories sparsely populated and the greater part of the folk of Rohan residing within the safety of the cities and holds and the lands south of the Snowbourne and Entwash Rivers.