After activating the DLC pack, both Challenge levels will be unlocked, regardless of how far you've gotten in the game.
This level is unique to this challenge map. While the others are closed off portions of the game's levels, this map contains a completely original area.
GRT Plaza is the large area in Genosha where you climb one of Sinister's towers.
The Tower feels completely out of place, resembling a medieval castle rather than a building in Genosha.