The Great Tower Stands in the center of the Nexus. You will unlock access to the main tower after leaving the first world, but to unlock the entrance on the first floor you will need red sigils that can be found in the next worlds. The tower consists of six floors. At the end of each floor, you will find a terminal which will help you activate further floors. The first five floors are locked with a puzzle from red sigils. Near the beginning of the game, Elohim makes a covenant with you never to ascend the tower, warning that it will lead to death for you and your generation.
The Talos Principle Wiki