 Because of the muddy <unk> of the site , work on the foundation continued past the work on the walls to <unk> . In <unk> , work on the first of the cathedral 's chapels began and by 1615 , the cathedral 's walls reached to about half of their final height . Construction of the interior of the current cathedral began in 1623 and what is now the vestry was where Mass was conducted after the first church was finally torn down . 
