stone steps, stone bridge
磴 centers on a stone structure for crossing or climbing: a stone bridge or a flight of stone steps.
磴 combines 石 (stone) with 登 (ascend), suggesting a stone structure for climbing or crossing. The exact historical development is uncertain, but the component relationship is clear.
Stone 石 on the left and ascend 登 on the right: picture a stone bridge or stone steps that help you ascend over water or up a slope.
For トウ, imagine a tall stone bridge with a tower at each end: tower -> トウ, and the stone steps lead you up to the tower.
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