Kautilya's Arthashastra 8.1.40

8.1.40

dṛśyate hi durgavatām anucchittir iti //

No, says Kautilya, for it is in the fort that the treasury and the army are safely kept, and it is from the fort that secret war (intrigue), control over one's partisans, the upkeep of the army, the reception of allies and the driving out of enemies and of wild tribes are successfully practised. In the absence of forts, the treasury is to the enemy, for it seems that for those who own forts, there is no destruction.