I have completed the civil war quest line with the stormcloaks and I was never sent to take Fort Sungard! I am thinking it may be from when I did "Season Unending" I had yet to join the stormcloaks and I had the empire give Markarth to the stormcloaks. Thus when I did start the civil war Fort Sungard was already under stormcloak control. If this the case then that is sad. If the stormcoaks already had control then I should have been given the acievement as soon as the civil war quest line was done.

I was thinking about using the console command to set the quest to the starting stage and them setting it to the complete stage and see if that gives me the achievement that I should already have!