Is there any point to the tunnel at Sudentor? It holds the SOLG control system, but that could've been put somewhere more convinient. Is the tunnel's only purpose to force the player to do an airstrike impossible ?
Like Pops said during the briefing, the real purpose of the Sudentor tunnel was to unite North and South Belka. The tunnel is a very streamlined path through the Waldreich Mountains, which might otherwise be too difficult to traverse. With this tunnel, the Grey Men could swipe supplies from North Belka as they had been during the entire game, transport them on cargo trains at the north end of the tunnel, and move them all the way to the south end of the tunnel into South Belka (North Osea). I don't doubt that this is exactly what they did to build all of those fortifications around the tunnel by the time ACES happened.