Why are Saint Thomas the apostle and Saint Barbara the patron saints of architects?

Barbara was imprisoned in a high tower by her father, DiOScorus, for disobedience. Her imprisonment led to her association with towers, then the construction and maintenance of them. This is a stretch, I know, but that is why Barbara is a patron saint of builders.

St. Thomas made a number of constructions while he was evangelizing in India including a palace he built for King Guduphara and the first church in India which be built with his own hands.