Total War basically lives up to its name. You'll be fighting at least one opponent throughout most of the campaign. It's intentionally like that. It's drip feeding you enemies.
If you go into it expecting Civilisation then you're going to be disappointed as it's not going to make sense. But when you see it as a game centred around military conquest then it's a lot better. It's all about context, basically.

Something I have never understood about the CIV games is how the AI players decide to place cities. Often they are too close to another one so there aren't enough resources which is especially frustrating when you have spent ages picking the right spot then whallop, the AI sticks a city right next door and it's a culture war.

In the first Civilization game the AI would cheat outright, sure they have only a couple of cities in the middle of nowhere with no resources and no irrigation/roads and out of a sudden WHAM! they build one of the Wonders of the World. Also getting military units every single turn as soon as you get one of your units near them.
I went and installed Freeciv just to try it out, and i have to say i'ts very well done clone for an open source project. I have a gigantic map and 23 opponents, and of course, i'm at war with pretty much every single one of them.
There's one civ that is fighting me tooth and nail over a few squares that form an isthmus, but those squares are full of resources. I guess the AI assigns a value to ressources and tries to keep them under their zone of influence as much as it can. The little bastards just won't give them up. There's another bastard that moved a ton of infantry units to the border and is just sitting there, i guess waiting for one of us to get weak. Not only that, but they used a break in the war to fortify their positions.
Someone must have put a lot of thought into the AI workings, not bad for a freeware game.

The best 4x games I have personally played is Alpha Centauri with expansion, Space Empires 4+5 though the normal non-modded AI is weak as a gummy bear, and GalCiv 2 which is impressive to me. Space Empires is based around warfare and fleets made much quicker for bigger battles, and is plenty of simple fun in a complicated format. GalCiv 2 is much more, fleets are much harder to make and diplomacy is an option for even warmongers.
Past that Sins of a Solar Empire Trinity pack is pretty sweet, though it's a sort of RTS with tons of thinking required and games can last hours.
EDIT: My linux PC has Freecol and Freeciv on it and those are great too, graphics not good but work fine. Freecolony and Freeciv are available for windows of course on top of Ubuntu.

Just wait a bit...I guarantee they'll release a new complete edition that includes Heir to the Throne.