diz, to answer your question, sometimes this situation occurs: You don't want population b/c of happiness You don't want workers/units b/c of gold/demographics/etc You haven't invented Research or Wealth yet Thus, the opportunity cost of pre-building stuff is zero, when compared to producing nothing, and it is only advantageous. You are correct, in that it if you need a worker, it is better to: 1. Build a worker 2. Build a unit as opposed to 1. Pre-build a unit 2. Build a worker 3. Build a unit However, the first situation does actually come up in games, especially for production-heavy cities, or on "quick" settings.