* There is now a sizable cluster penalty on fortresses, making them not very useful when clumped together too much on one planet.
Sizable? Yes. Extreme? YES.
My typical frontline defense has four fortresses. To pull this off now, one of those marks has to have their attack power reduced to 20% (with two of them, that's the equivalent of a single fortress running at 40% capacity -- less than half) and I have to spend 12,000 knowledge where I used to only have to spend 4,000. So I spend 12,000 knowledge for less power than I used to get with 4,000. There's something terribly wrong with this picture.
My goal is to nerf exactly that, and I know it's an extreme change. You can still have three fortresses at full power, but they have to be a mark I, mark II, and mark III together (the reduction is per-mark-level, as with sentinel frigates, etc). As for the other mark Is and IIs, you'll have them free for using on other planets.
I'm not terribly worried about this picture at all, you aren't meant to have so much concentrated firepower all at one place. This is always the problem I face: the more ships I give people, the more they like to stick them all in one big blob that doesn't have much in the way of strategy associated with it. My continual job is finding new and clever ways to stop that, heh.
In this specific case, you get a lot for spending a lower amount of knowledge for the mark I fortresses, but then you have the opportunity to get the mark II and III ones if you
really want to specialize in fortresses. As with many (but not all) specializations, you get somewhat diminishing returns for increasing costs. In this case, the mark III fortress is absolutely a powerhouse, but you only get the one and it's pretty pricey.
Having a mark I, mark II, and mark III fortress would actually be more powerful than your old thing of having just four "no mark" fortresses in the past versions, I'm fairly sure (though I've not run the numbers). You would spend a lot more knowledge on that, but I feel like that amount of knowledge is commensurate with the benefit you're getting out of it.