I used an Empty Keep map from the Roll20 Marketplace for this.... in my past campaign of AD&D 2E... It's a map that just has the Walls and Gates of the Keep, with just blank grass on the inside. Then my DM let each of the Players type or draw (Roll20 drawing tools) onto the grassy area, saying what their PC would build within the Keep. For example the cleric built a temple and a brewery, and a horse barn. The druid made a wooded copse. The fighters made tall guard towers, and training grounds. This was a long time ago 2017 on Roll20 and I just thought of it to share the idea.