There is however still no plausible explanation for why the earthen rampart and ditch built around Pliska encircled such a large area. A few areas appear to have been inhabited by people who provided services or took part in craft production for the palace centre, and agriculture clearly took place within the earthen rampart around the Outer Town. There was clearly a fair amount of open ground inside the earthen rampart. Henning concludes that the settlement was initially built for military and strategic reasons, and could not really be considered a typical city (with all of the associated trade, production, and population) until the period after Bulgaria converted to Christianity in 864/5. Most of the stone architecture in Pliska was built between this conversion and eventual Byzantine conquest of the city in 971. The buildings from this era show a clear Byzantine influence in their design and function. For example, a 4 kilometre-long canal fed a large brick cistern that moved water from the Kriva river to the Inner Town via ceramic pipes. This cistern had attached rooms containing private baths.