bank, dike, protuberance
埝 centers on raised earth structures: a bank, dike, or any protuberance in the ground. The meanings all describe a mound or ridge of earth.
埝 combines 土 (earth) with 念, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests an earthen mound or bank.
The left side is earth 土, and the right side 念 looks like a thought rising up. Picture a raised bank of earth that stands out like a thought in the mind.
For テン, imagine ten earth mounds lined up like a dike: ten -> テン, and each mound is a protuberance of soil.