The rajas of mahajanapadas built forts in order to protect their capital city and their people from attacks by other rajas. It is also likely that they wanted to show how rich and powerful they were by building large, powerful and impressive walls around their cities. Another prominent reason could be that it was easier to control the land and the people living inside the fortified cities

The rajas of Mahajanapadas built forts for various reasons:

1. Defense: The primary reason for building forts was to protect their kingdoms from external attacks. The Mahajanapadas were often involved in conflicts and wars with neighboring kingdoms. Building forts provided a strategic advantage, as they served as military strongholds from where the rulers could defend their territories.

2. Security: Forts were also built to ensure the safety of the royal family and the ruling elite. They provided a safe haven during times of unrest, rebellion, or invasion. The fortified walls and structures offered protection against enemy forces, making it difficult for them to breach the defenses and capture the rulers.

3. Administration: Forts served as administrative centers of the Mahajanapadas. They were the hub of governance, where the rajas and their officials carried out the day-to-day administration of the kingdom. The forts housed administrative offices, courts, and storerooms for the collection and distribution of taxes and resources.

4. Symbol of power and prestige: Building forts was also a way for the rajas to display their power and authority. The grandeur and size of the forts reflected the wealth and status of the rulers. It showcased their ability to mobilize resources and manpower for such architectural marvels, thus enhancing their prestige and influence among their subjects and neighboring kingdoms.

Overall, the construction of forts by the rajas of Mahajanapadas served as a means of defense, security, administration, and displaying their power and prestige.