I haven't tried exploiting citadels yet. Once the AI gets in the mood for war, I build a massive force and steamroll them. The 15% combat bonus that Great Generals provide don't seem to stack, so I've just been keeping them spread out so my front line units receive the bonus. It seems that if I use the General to build a citadel, I'll lose the bonus. It doesn't take longer than half a dozen turns for me to cap a city. I don't see the point to taking the General off the field to block off an area that will soon be behind the front lines. Perhaps if I was fighting more of a stalemate, I might utilize the citadel. In an even match, it seems like it could be an easy way to all but guarantee a victory.