This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
壂 combines 殿 (hall, palace) with 土 (earth). The exact historical development is uncertain, but it likely relates to a type of earthen structure or foundation associated with a hall.
With 殿 (palace) over 土 (earth), picture a grand hall built on a raised earthen platform. The kanji might suggest a foundation or a type of building, but its exact meaning is obscure.
For テン, imagine the number ten (テン) earthen platforms supporting a palace: ten -> テン, and the palace sits on the earth.