You need special building that Durthu have. If you dont have such building check your mods, unlocker mods remove custom buildings.

引用自 Venris：

You need special building that Durthu have. If you dont have such building check your mods, unlocker mods remove custom buildings.

I have several of the special building, I still can't figure out how link strength works. I assume it's some combination of the level of the capitals, the levels of the worldroot buildings, nearby enemy settlements, whether or not neighboring capitols are controlled by AIs or you, some number of hidden or random variables, maybe?...but none of that actually tells me how it works. My capitol is lvl 5, with a lvl 4 worldroots and two other capitols with world roots adjacent, but it still won't go above link strength 2.