Well, this is what happens on the bigger green square in a province. And you can move your guild HQ whereever you want. However, there could be a smaller green field. In some provinces (noman's lands) there is a limitted number (4) of outpost slots. When one of them is occupied, you can see the outpost and the outpost's owner guild. When all 4 are taken- well, you have to destroy one of them in order to build yours there. In other provinces (dangerous and safe lands from tier 2 or higher) have unlimitted outpost slots. In this case, you can always see one free slot and colonise it (but you cannot see other guilds outposts). Finally, in Hope Village Plains province you cannot build an outpost at all.