In the 10th century the settlement assumed an important strategic role: as part of the protective chain along the Ipoly river, it defended the most important crossing of the river. When forming the royal counties, the population of the villages which carried out military tasks was entered the royal castles' services, so Gyarmat became part of the Hont castle.

In the summer of 1552 the castle of Gyarmat left alone by its protectors was invaded and devastated by the Turks. This invasion lasted till 1593. At the beginning of the Fifteen-Year-War, in the autumn of 1593 the Turkish guard broke away and set the fortress on fire. So the border fortresses were coltrolled by the hungarian royal army again, however the reconstruction of the fortresses started only in 1602 according to the parliament's decision.

It was Forgách Ádám the captain of Érsekújvár, who chased out the Turks definitely. Balassagyarmat ordered a painting for the memory of the victory and the women. In 1908 a monument was stood in the Paloc park for the memory of Balassagyarmat's heroines.
In the years of 1650 Balassagyarmat was the center of an estate. 19 bigger and 7 smaller settlements belonged to it. Fron 1652 Ferenc and Imre Balassa became the captain of the castle, so they could practice their landowner and comissioner right at the same time.

Balassagyarmat near the 18th century border castle had an excellent geographical position, it lay at the meeting point of important roads, so it became a Fair place in the 14th century.
The route in the Ipoly valley lead to the mining towns of high Hungary. This was protected by Gyarmat, the fortress. Another important route lead through Gyarmat in the direction of Alföd which served the exchange of goods between the people of the two territory.
In the fairs held 5 times a year a lot of people took part, and the market towns got the fairs' income.

The border fortress of Gyarmat was invaded again by the Turks in about 1663.
According to certain sources the army of Apafi Mihály Prince of Transylvania exploded the castle. Other informations said that in 1665 in the fortress of Gyarmat hungarian gallant soldiers garrisoned and afterwards it was ruined. With this Gyarmat lost it's fortress character once and all and became an uninhabited field. One part of the population settled in the nearby villages, other parts moved farer. The soldiers were allowed to work for other fortresses.

In 1790 Balassagyarmat became the county seat, that had a great importance in the town's progress. Balassagyarmat's function as the county seat was permanent, Salgótarján took its part only after 150 years. Previously at the beginning of the 18th century the county's assembly took place at Losonc and Szécsény. During the reign of Mária Terézia in 1754 it was decided again that the permanent county hall would be in Balassagyarmat. The relations of the market-town didn't allow the realization of the plan. So the new county hall was built in the village of Szügy not far from Gyarmat. Between 1763 and 1790 it was Szügy where the county's assemblies, the court of law and the county archives took place. In 1790 according to Nagy Iván the county managed to get in Balassagyarmat into a building which was bought from the state. The county directed its subjects from this so called "small county hall" till 1835 when the new building - that influenced the sight of the town nowadays, too - was inaugurated with all due ceremony on 19th October. The ecomony of Balassagyarmat came to life from the years of 1820 after became the county seat. After the French wars Balassagyarmat became the most important place in the whole Ipoly valley. Its industry and commerce prospered because of its function as a county hall. From the years of 1820 the towns was a significant political forum for the noblemen.

As the noblemen was flowing into the towns it was necessary to extand new industrial branches and to renew some commercial forms. The most important scene of the commercial life was the fairs. Country-wide fairs were kept 8 times a year and the weekly fairs' days were Monday and Friday. In the Reform Era Balassagyarmat's famous grain and consumer goods fairs attracted not only people from the neighbourhood but tradesmen and merchants came from the farer regions at the same time.
Besides country-wide fairs the new specified permanent shops got a bigger and bigger role.
According to the data from 1828, in Balassagyarmat there were 51 merchants selling products and 42 questors selling raw materials, that was the 38,27 percent of the whole county's merchants. The town was the 21st largest and most important free market town of the country thanks to the high number of merchants living in Balassagyarmat. The merchants of the towns consisted of 3 ethnic groups: Greeks, Germans and Jew.
The Greeks and the Germans brought into fashion the first specialized shops. The German merchants were mostly involved in the trade of iron products. However most of the merchants in the county seat were Jews, they mainly were interested in trading hide and wool as well as grain and crops. They were the members of the Jews' well-to-do stratum. However the other part of the Jewish merchants were more poor.

By the end of the Reform Era Balassagyarmat was outstanding among the market-towns of the counties with its 7529 strong population.

Balassagyarmat in the Reform Era showed a rather colourful picture concerning the ethnic groups and the religions. The town was inhabited by catholic hungarians and lutheran Slovaks from 1690. The peasantry spoke slovakian and among the tradesmen the hungarian and the german language dominated. From the years 1730 Greek Orthodox and Jewish merchants moved into the town which meant a wider range of ethnical and relgious composition. In 1825 concerning the population the town housed the 13th largest Jewish community with 870 members.

In the years 1840 the Jewish could move into the minding towns and their neighbourhood that was prohibited earlier. So the Jewish community in Balassagyarmat was close to 2000 at about 1848.

The revolution town administration was founded in 25. March 1848, at the same time they decided to establish a national guard in Balassagyarmat and in other market-towns of the county in order to prevent the arising movements.

Between 1850 and 1870 decisive changes occured in the civil society and economy. Balassagyarmat had two hugh tasks in order to create its civilian future in the decades of the dualism: first of all to develop an ordered town and to build the railway in the Ipoly valley.
In 1886. the settlement lost its market-town title. The town's administration was not able to provide neither the institutional conditions nor the payment of the apparatus.
Analyzing the town's development it had a beneficial influence that Losonc and Balassagyarmat was compared in the question of selecting a new county seat, so the administraton hurried to provide the suitable conditions for being a town again. Balassagyarmat had more and more institutions, unions and cultural organizations.
The traffic terms of the town was rather bad, it hadn't any railway, roads with the exception of the Kassa Road which condition was acceptable.
By the turn of the century some change occured in the condition of the public road network.

The urbanization reflected by the settlement's outlook was seen by the end of the 19th century. The planed, town developing politic reasulted important changes in the structure of the settlement. New districts and new roads built in the town. The public buildings on the main street made an urbanized outlook, by the new house building strategy made a regular street-system.

At the end of the last century built the Credit Bank of Balassagyarmat, the Mária Valéria Hospital of Nograd County. It was Wälder Gyula architechture (later teacher at the Technical University of Budapest) who had a large part in forming the new, small town face of Balassagyarmat.