Question

How is a barrel vault constructed?

And
Why were buildings constructed with barrel vaults dark and
gloomy inside?

Answer 1

A barrel vault is a continuous arched shape that may approximate a semi-cylinder in form, resembling the roof of a tunnel. It is formed by a series of arches or vaults placed side by side or by a continuous shell

Builders were reluctant to pierce the thick walls of vaulted buildings with windows,they feared that the openings would weaken the vault