I wonder why the Lannisters and the Tyrells haven't sought out Stannis and thought to finish him. Dragonstone isn't far from King's Landing, and as far as I know Stannis hasn't officially surrendered or yielded his claim, thus making him an enemy still...
It's an island fortress surrounded by the sea, it wouldn't take very many men to man the walls and make it extremely difficult to capture. I'm sure it was said at some point in season 2 (when they introduced Stannis) that Dragonstone's walls had never been breached either.
In the last episode when Stannis said the meat was off, perhaps the crown has blockaded Dragonstone preventing resupply. It would be the best way to capture the citadel even if it would take years.