heap, firing mound
垜 centers on piling up earth or material into a raised mound, especially for firing or military purposes. The heap sense is a natural extension of that piled shape.
垜 combines 土 (earth) with 朶, which likely contributes the sound and the idea of something piled or drooping. The exact historical development is uncertain.
The left side is earth 土, and the right side 朶 looks like a pile of things stacked up. Picture a mound of earth heaped up for a firing range.
For ダ, imagine a dad building a dirt mound in the backyard: dad -> ダ, and he heaps the earth into a firing mound.
mound on which targets are placed (in archery); firing mound