small mound
塿 centers on a small mound or hillock of earth. It refers to a modest raised pile of soil, not a large mountain.
塿 combines 土 (earth) with 婁, which likely contributes the sound. The character is rare and its exact historical development is uncertain.
The left side is 土 (earth), and the right side 婁 looks like a piled-up structure. Picture a small heap of soil shaped like a tiny mound.
For ロウ, imagine a low mound of earth: low -> ロウ, and the mound is small and low to the ground.