This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
垌 combines 土 (earth) with 同, which likely contributes sound. The exact historical development is uncertain.
The left side is earth 土, and the right side is 同. Without a clear meaning, picture a mound of earth that looks the same (同) from every angle.
For トウ, imagine a toe (トウ) poking into the earth (土) beside the same (同) mound: toe -> トウ.