Small amounts of water (e.g. surface run-off or boggy ground) were dealt with by means of drainage channels in the base of the curtain wall. Permanently running water would be channelled through culverts, like one noted at Denton Burn by Collingwood Bruce. Major rivers, such as the Eden, Irthing, and North Tyne were crossed by bridges large enough to carry a walkway and, later, even a road. What is not known is how the openings were blocked to prevent intruders getting through, although some sort of wooden or iron grille is a possibility.