The position is strong, being protected by the two rivers mentioned, and the medieval fortifications, which are nearly 2 m.
The waters of the two rivers have, however, been made to communicate by artificial cuts and canals in more than one place.
At this point also the two rivers are connected by a canal, the northernmost of a series of canals which formerly united the two great waterways, and at the same time irrigated the intervening plain.
To keep these in check, Gilbert de Clare, during the closing years of the reign of Henry III., built the castle of Caerphilly on the southern edge of this district, in a wide plain between the two rivers.
It naturally falls into two divisions, the northern being more or less mountainous, while the southern is flat and marshy; the near approach of the two rivers to one another, at a spot where the undulating plateau of the north sinks suddenly into the Babylonian alluvium, tends to separate them still more completely.
