There are many useful Kingdom websites around the internet and some of them are not in English. I will list some here for and you can just take a look even if you don’t understand the language. I might add more in the future.
Main scanlator for the English version of Kingdom. Kingdom’s popularity in the English world would not exist without them. Do visit them for the latest English chapter and thank them for their efforts if you wish to.
Chinese Kingdom Forums
The Chinese people call it “tieba”(贴吧), it is quite similar to the reddit we know in the English world where there are many sub-forums within it, Kingdom being one of them. This is where all the latest Chinese scanlation of Kingdom are usually posted. Many vital information(such as interviews, release date of Kingdom games etc.) can also be found there with all sorts of general discussions related to Kingdom. Since i do understand Chinese, I will usually check out the forums on a regular basis and update the English community, both here and mangafox, if there are any noteworthy information posted there. If you love hitting F5 while waiting for the latest Chinese chapter of Kingdom to be out(usually between Saturdays and Tuesdays), this is the place to visit. You can also visit other sub-forums using the search bar there but you do need to type in the Chinese name of the sub-forum to find it.
Official Kingdom Guidebook
Wonder how strong each and every character in the Kingdom universe are? Want to know more about Kingdom from the author’s perspective? The official guidebook contains all the answers…except no one has translated it into a non-Japanese language so you will need to understand Japanese to understand most of the contents…
Kingdom Fanart and Anime pictures
Contains nice Fanarts and screenshots from the Kingdom anime for some of the more important characters. You can check it out if you need some pictures for your wallpaper. And yes…Kingdom had 2 seasons of anime…though I will advice you not to watch it as the animation don’t do the manga justice as they probably used up all their budget on the seiyuu cast (google the anime if you are interested).
Mr Hara Yasuhisa’s own twitter where he updates his personal status and tweet Kingdom related stuff. It is in Japanese though so you might need some form of translation to understand.