It's a silly question I know but how can I play the expansion? In the game menu it says the expansion is installed but I cannot find a way to play it directly. Is it only accessible from within the normal gameplay? I use the GOG version.
I think so. It's more content in the main game you can go to during the main plot after you get the stronghold (more or less).
It's not a continuation past main game plot.
Are there any requirements other than getting the stronghold? I started a new game and have the stronghold now but there doesn't seem to be a way to access the new content