mound, tumulus, cylinder, fort
壔 centers on a raised earthen structure: a mound, tumulus, or fortification built from piled earth. The cylinder sense likely comes from the rounded shape of such mounds.
壔 combines 土 (earth) with 壽, which likely contributes the sound トウ and may relate to an old word for a mound or fortification. The exact historical development is uncertain.
The left side is earth 土, and the right side 壽 looks like a tall, layered structure. Picture a mound of earth piled up into a rounded fort or tumulus.
For トウ, imagine a tall earthen tower: tower -> トウ, and the mound rises like a small tower.
prism