土 depicts a mound of soil. As radical it places 坞 in the earthwork family - built structures of dirt and stone. Joins 城堡墙, all defensive or built-up sites raised from the ground.
乌 supplies the sound (wū drifted to wù). Originally a pictograph of a black bird, here purely phonetic. The same wu phonetic appears in 邬钨呜. The earth-radical anchors meaning while 乌 carries the sound.
