Baron Nashor

Sharp eyed readers may remember that Baron Nashor doesn't work this way. He enters at the Baron Pit regardless of effects preventing him from being played to battlefields. The reason for this is that Baron Nashor's passive ability generates a delayed replacement effect, the one that reads "I enter there." This replacement effect replaces the event of him entering to any other location with him entering to the Baron Pit. The replacement effect gets around effects like Mageseeker Warden which limit where you can play units. However, Dragon Roost is not written as a replacement effect. It tells you where to play the dragon you've paid the additional cost for, but it doesn't specify how that dragon enters the board. Therefore it is a permission on play location, but not a replacement effect.

369.3. Replacement Effects that apply to a unit as it enters the Board can be identified by describing how the unit enters, or by describing a game action that occurs "as" a unit enters. Example: Master Yi, Honed reads "I enter ready." This applies a replacement effect to the way that units normally enter. The event of him entering exhausted is replaced by one where he enters ready. Example: Baron Nashor reads "As you play me, add the Baron Pit battlefield token to the board if it's not there already. If you do, I enter there." The last sentence of his ability is a replacement effect that replaces the event of him entering at his original play location with him entering at the Baron Pit if it was created.