This moat, while impressive, would not have stopped a determined attacker. People sometimes wonder why Kasyapa accepted Mogallana's challenge to open battle - a fight that resulted in Kasyapa's death - instead of just hunkering down on top of Sigiriya Rock. The answer seems to be that Sigiriya was tactically impregnible, but strategically vulnerable; eventually, as at Masada, defenders on top of the rock would have run out of basic necessities like food and water, while the attackers would have taken over all the resources of the lower city and surrounding countryside.