[dogscoff]

But it does seem a little wierd that you cannot upgrade to a cultural centre. I suppose the SE4 game structure prevents it in some way.

Yeah- because cultural centre tech is available from the start of the game (it has to be, because that's what your homeworld starts with), it would break the city upgrade system if it were on the upgrade tree.

The game only upgrades to the highest thing on the tree, so if you build a minor city and have city/major city tech availabel, you would ONLY be able to upgrade it to to a major city- you CANNOT upgrade to the intermediate size (city). That's why the colonial development techs are so cheap - they are only there to allow you some control over your upgrades (ie don't research major city until you have issued all your "upgrade to city" orders)

Likewise, if cutlural centres were at the top of that tree, you'd ONLY be able to upgrade to them, you'd never have the option to upgrade to to a city or major city or any other intermediate stage. Because CCs are so horrendously expensive, this would that whole upgrade path utterly unusable.

Of course the (seemingly) obvious solution is to make CCs affordable, but that's just not how proportions is supposed to be. There are a few long-winded and occasionally heated threads around here as to why they should be priced as they are.