hill, uphill path
嶝 centers on a steep or rising path on a hill: a hill itself or an uphill path that climbs it.
嶝 combines 山 (mountain) with 登 (climb, ascend), suggesting a hill or a path that climbs a mountain. The exact historical development is uncertain, but the component relationship is clear.
A mountain 山 with a climbing path 登: picture a steep hill with a winding uphill path carved into its side.
For トウ, imagine a toe (トウ) stubbing against a rock on the uphill path: toe -> トウ, and the hill makes the climb painful.