dike, embankment
塘 centers on a raised barrier of earth used to hold back water: a dike or embankment.
塘 combines 土 (earth) with 唐, which likely contributes the sound. The character expresses an earthen structure for water control.
The left side is earth 土, and the right side 唐 suggests a raised shape. Picture a long mound of earth built up to hold back a river: an embankment.
For トウ, imagine a tall earthen dike that looks like a tower: tower -> トウ, and the earth mound rises high like a tower.