Where the shape comes from
A roof built out against a cliff or a standing wall, one side left open: the dot is the ridge, the horizontal the eaves, and the long sweeping stroke the wall running down. It heads rooms and buildings — 店, 座, 床, 庭, 库. Simplified writing then took the same shape over to write 廣 guǎng "wide", as in 广场 and 广告.