We've plugged Yar's escape route, as well as other fissures, towers, and side-doors, but our problem is the Wyvern Caves on Zilfin. They haven't actually held Wyverns since the days of the ancestors, when Grimnir slew them and they fed him and Grungni and Valaya during their work here, but they are a massive network of caves, too massive to block off in the time we have. We could take a heavy toll off them trying to climb up build their way up it, but we'd also be exposed to anything they might have in the way of siege weapons or other surprises. It's also below the point the Ice Dragon has 'claimed'.