I finally conquered my first colony island and got 2 more quests - one under the other - which so far means that they can be performed independently from each other in random order: Build your defence and Claim the colony. So I decided that the second one will be done faster and made it first. Then the colony rectangle on the right side disappeared. Now I don't see from where to go to my new colony to build defence on it. Anybody can provide me with a hint?