Here's a video on how to obtain ALL of the prologue achievements, and more! Links to the specific achievements, in order of when you'll obtain them, are in the video description. I didn't show battles as some are walked through by the AI, and most others are simply auto fought and won. The last one appears very difficult, but my game cut out, and I didn't complete the prologue.
As for this achievement:
1. Once you get to Loki Palace, whether you've built or not, 5 buildings should be within reach.
2. You require population stability within each fort/outpost/settlement, which is gained by building secondary buildings that have a "population boost".
3. When turns pass, the population goes up. You will need to build up your main settlement building before building secondary buildings. Build 5 buildings overall between Loki Palace, Fort Devingard, and your other bases, and the achievement is yours.