Mageseeker Warden

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.

358.3.a. If a Game Effect prevents the performance of a game action, that effect doesn't prevent cards and abilities that instruct a player to perform that game action from being played or finalized. On resolution, that game action will be skipped as it is an impossible instruction. Example: A player plays Here to Help when their opponent controls a Mageseeker Warden at a battlefield. Here to Help is legal to play under these circumstances. On resolution, no unit will be played.