Cannons continued to be improved during the 15th and 16th centuries. Castle loopholes were adapted to allow cannons and other firearms to be used in a defensive role, but offensively gunpowder weapons still remained relatively unreliable. England had lagged behind Europe in adapting to this new form of warfare; Dartmouth and Kingswear Castles, built in the 1490s to defend the River Dart, and Bayard's Cover, designed in 1510 to defend Dartmouth harbour itself, were amongst the few English castles designed in the continental style during the period, and even these lagged behind the cutting edge of European design. Scottish castles were more advanced in this regard, partially as a result of the stronger French architectural influences. Ravenscraig Castle in Scotland, for example, was an early attempt in the 1460s to deploy a combination of "letter box" gun-ports and low-curved stone towers for artillery weapons. These letter box gun-ports, common in mainland Europe, rapidly spread across Scotland but were rarely used in England during the 15th century. Scotland also led the way in adopting the new caponier design for castle ditches, as constructed at Craignethan Castle.