To open the door: Shoot the enemies running through the area in the correct sequence.
To activate the enemies: Find the central platform and observe the activation order displayed on the central screen.
Central screen: A diegetic UI that generates a password sequence and a timer when activated by pressing the E key. After a 3-second timer, the screen disappears and the enemies are activated.

Create the UI to display the three options: square, circle, and triangle.
When colliding with the platform in the center (the HUD later), open the UI with the password.
Count down from 3 and then close it.
Show the timer (so the player can prepare for the next-gameplay-step, the shooting).