Good point about ceding castles, I went around and did that with mine, which has bumped us up a lot with land control. Thanks, that was a point I'd missed, so I updated the OP with the information. It's important that people do this to grow the land claim areas.
We now have a land area in excess of Cardolan, and our Majesty is only 2 points shy of Cardolan's too. I'm sure this will bump over the line very soon. 20 cities fall under our domains, plus at least 1 more from a bay12er who is yet to pledge alleigence.
As for the green forts, they'd be safe to leave alone if it was just Green, but the biggest risk is that Stormtroopers takes them over and sticks Red garrisons in there. That might become more of a realistic threat if Red views us as a threat. It's probably a good idea to grab them, since Skr won't actually retaliate. He barely has enough guys to man the 3 fortresses around his home city. Also, we've removed his sole reason for extending to the South now, so he should lose interest in that direction. He is back down to 1 controlled city because his empty castles are no longer claiming any cities.
So, if you can commit to manning one of those empty green fortresses go and grab one, but declare in the thread here that you're doing so.
I'm gradually building up some more castles, but it takes time to level rookie garrisons up enough that I can move my party onto the next one. It's the easiest way to grab more cities though. I'm going to fill in that gap in the South with blue, over the next couple of weeks.