The mine was where you got a cutscene earlier in the game of them blowing up the entrance if you approached, and Talana gave a warning that they were too strong and you should not enter. The other mine entrance should be marked on your map, south east of the lake, and it is only a short flight between the mine and tower.
The mine tower and the tops of the walls remain above the poison gas, as does the top levels of Lovis' tower and a couple other places.
