wall, fortified wall, small wall
墉 centers on a constructed barrier: a wall, especially a fortified or defensive wall. The sense of a small wall is a natural extension of the same idea.
墉 combines 土 (earth) with 庸, which likely contributes the sound. The character expresses a wall made of earth or a fortified earthen structure.
The left side 土 is earth, and the right side 庸 looks like a sturdy structure. Picture an earthen wall built strong and fortified.
For ヨウ, imagine a wall so tall you have to yell 'Yo!' to be heard over it: Yo -> ヨウ, and the wall echoes back.