How do we know Colchester Castle was built before The White Tower in London?
Type Colchester Castle and The White Tower (of London) into google and read the information from Wiki. According to Wiki, building of Colchester Castle started between 1069 and 1076 and was completed around 1100. Again according to Wiki, the White Tower of the Tower of London was designed based on Colchester Castle and building commenced in 1078 but was completed in 1087 (before Colchester Castle was completed).