Genshi Otome to Kami no Tou

Set in a post-apocalyptic future, humanity lives in a vast jungle with stone age technology, hunting animals like bears and mammoths for food. The landscape is dominated by a gigantic "Tower of God" (the ruins of Tokyo Tower), said to be where the treasures of a past world were hidden after the previous world was destroyed by the gods.

(Source: Prehistoric Fiction Wiki)