I have tried sneaking around from Southfringe Sanctum thru the mountains to the lake and the damn Imperials still spot me. How do I defeat their radar? I'm even wearing the Nightingale Set of Armor...

Invisibility could work.

Scout the area BEFORE you accept the quest will show you where you could go.

And there's my favourite: Forget sneaking. Simply attack the Imperials in the Fort.

I AM invisible, I'm coming over the mountains from the east, staying away from the roads

If I remember right (always a question) I think that fort is one of the places that you have to get into through some underground entrance as part of a quest if you want to be undetected... Scripting sets NPCs to detect you at key spots and distances to prevent you from just sneaking in.