Complex sentence: In Valdemar's Danish Census Book from 1231 it was called "Aleburgh", possibly meaning "The fort by the stream" as "all" in Old Norse meant a stream or current and "bur" or "burgh" a fort or a castle.
Split into simpler sentences: In Valdemar's Danish Census Book from 1231 it is called "Aleburgh". The name then possibly means "The fort by the stream" as "all" in Old Norse means a stream or current and "bur" or "burgh" means a fort or a castle.