I'm building a gaming pc and that's one of the game I'd like to give a try: is it an RTS or is it more like a city builder?

More like a city builder with some locked mechanics on economics, but also includes scenarios you can plan, which add other non-normal modes to the game.

The version I have enjoyed most were the ones you could build over and under water. Forgot which version was that one... but that one was the "amazing" one for me in terms of gameplay. But its not recent, so graphics might not be the ideal ones anymore for people looking for some appealing stuff.