Mageseeker Warden

Q: My enemy has a <<Mageseeker Warden>> on a battlefield. I attempt to play <<Baron Nashor>>. Assuming a Baron Pit doesn't exist yet, is <<Baron Nashor>> legal to play? If so, where does <<Baron Nashor>> go?

Rules for Context - Card text of “As you play me” not defined

A: It is legal. Baron’s effect is a replacement effect. It replaces the event of him entering to the location he was played to, with him entering the Baron Pit.