Proposed Canon
The Faq Gap is a break of Mountains or High Mountains in the ring of Impassable Mountains surrounding the Kingdom of Faq. (Described by Vinny Doombats as "the only passable hex in High Mountains.") Through this hex, a group of flying units and possibly mountain-capable units may enter or exit the kingdom. It forms a natural choke point into and out of the kingdom. (Another being the Tunnels through the mountains).
Impassable Mountains are a Terrain Type which cannot be passed even by flying Units. If fliers from Transylvito used to pass through Faq airspace within visual range of Faq's cities, on the way to somewhere else, then there must be at least two places in the Mountains surrounding Faq where fliers can pass.
The question, then, is whether Transylvito units regularly passed through the actual Kingdom of Faq, or merely near it, and if they regularly entered the kingdom while travelling, or merely investigated it once or twice, finding nothing thanks to Jack Snipe. Or if they flew to the tunnels, walked through them, then explored the area and went back. If there was nothing there but mountains surrounded by high mountains, why would Transylvito units pass through at all, rather than going around? Being that Faq was a small kingdom, it could be circumvented without even really noticing.
If there is another passable hex, it begs the questions, why is it never referred to and why was it not defended when Jillian set her trap for Stanley?