Regarding the bonus trade route from the Citadel, I thought all castles gave a bonus trade route anyway? If so, you can't really claim it as part of the Citadel's specific benefit. It's true that you'd be more likely to want to build a Citadel than a castle, so in practice they'll give you more trade routes, but in principle you could get the same effect from castles if only they were worth building in their own right.
I liked Spain in Warlords. Isabella suited my playstyle. Of course, I'm really bad at the game, so this certainly doesn't indicate that she's a decent leader, but I got on well with her. I'd have preferred industrious instead of expansive, but I love spiritual. If you grab some stone beeline to her UB, it arrives on the scene at exactly the moment when I've satisifed most of my lame 'builder' tendancies and started preparing to take out a neighbour. You also get the UU at about the same time; for a while, you're unstoppable. It goes without saying that you delay getting economics for as long as possible. Churn out a supply of siege weapons while running vassalage and theocracy (thank you, spiritual trait!) and keep them for later, then go ahead and grab economics.
It's a shame that both the UU and the UB got nerfed in BtS. The UU is a bit of a joke now (now you see it, now you don't!), and the UB suffers from the siege weapon nerf. Expansive has been given a slight boost (in the form of massive late-game health penalties, which it helps to make up for), but all in all Spain have gone from barely average to terrible.