This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The historical origin of 牚 is uncertain. It visually combines a 尚-like top with 牙 at the bottom, but the component relationship and original meaning are unclear.
The top looks like a roof or cover, and the bottom is a tusk 牙. Picture a pillar (はしら) made of tusks holding up a roof, though the meaning is uncertain.
For トウ, imagine a tall pillar (はしら) shaped like a tusk 牙, and think of a towering (トウ) structure: tower -> トウ.