I have a question regarding beavers

Now I see beaver marks on trees here and if I remember correctly I mer one or two As well. But they Don't seem to be constructing any dams neither those beaver castles (maybe they are called something else ín english) . For reference there is an artificial Dam here already and they are european species if that matters. Do all beavers construct these domeciles and dams or do some of them have alternative lifestyles . I also saw an otter today he ran fast