historical tower of 'ponts couvert' bridge as part of defensive work on the river ill in 'petite

famous strasbourg river iii tower ponts couverts france bridge place sight architecture building city defence defensive work europe european historic historical landmark monument old petite quarter couvert protection ill stone tourism tourist town travel winter 'ponts couvert' erected 13th century 'petite france' castle waterway reflection water monastery fortification estate palace tree ruins canal