Help with city building game?

So I have been making games for a while, and I think I am ready to step it up. I want to make a simple city building game, so where should I start? (Besides modeling buildings, terrain)? Can anybody help?

I think a good place to start would be implementing building placement which checks if theres a building already there (people ask this a lot in relation to tower defence games, so a quick search should sort you). You’ll probably want to maintain a list of placed buildings so you don’t have to search for them when you implement logic.

If you need something more specific, I would recommend revising your question.