money repository
帑 centers on a place where money is stored: a treasury or money repository.
帑 combines 奴 (which may contribute a phonetic element) with 巾 (cloth), possibly suggesting a cloth bag or wrapping used for storing money. The exact historical development is uncertain.
奴 (servant) over 巾 (cloth) — imagine a servant wrapping coins in a cloth to store them in a treasury.
For トウ, picture a tall tower (タワー -> トウ) where the money repository is kept safe.