shut as tight as the doors of the Radley Place

This section of the sentence is symbolising the connection between the townspeople and the Radley's; the Radley's closed Boo off from the town just like the town is closing off Negro people. The Radley's and the townspeople both have this narrow mindedness that stops them from accepting each other. Boo is different to the rest of Maycomb folk and because of this he is treated like an outcast, a freak. People fear him, they fear the house of the Radley's, some are even afraid to walk past their house.