fall in ruins, collapse
塌 centers on structural failure: falling into ruins or collapsing, as when a building or land gives way.
塌 combines 土 (earth) with a phonetic element that historically included 日 and 羽, suggesting earth collapsing or falling. The exact historical development of the right side is uncertain.
The left side is earth 土, and the right side looks like a structure with a roof, a sun 日, and feathers 羽. Imagine an earthen building where the roof and walls collapse into a pile of dirt and feathers.
For トウ, picture a tall tower (トウ) of earth that suddenly collapses into ruins: tower -> トウ, and the earth gives way.