The final plateau is an unfinished area and also the site of a final boss battle. Currently, there is only a sign that states, "Final boss here. You win!"
As of Version 2.3.7, no monsters naturally spawn on the final plateau. The final plateau appears as a large flat stone area with a massive castle atop it, with dozens of spires piercing the clouds. The area does need to be lit as there are no natural light sources within the castle.