What’s the deal with ‘collision’ when using the castle planner?

I’m having trouble diagnosing my issues when ‘commit’ is greyed out, stating ‘collision’ as the reason. I see that some stuff on the map glows red, but I can’t figure out exactly what it is trying to tell me. The gatehouses are especially confusing. Is it an elevation or steepness problem? Let me know your basic best practices for basic castle planning.