You know, this game would definitely benefit from having some kind of manual. I imagine the xp requirements for levelling up vary by unit and are a mystery to the player until it happens? Is an undefended city taken automatically when invaded or does it get some kind of native defence?
Yeah, a manual would be a great help indeed. I might actually consider making one if I get the time.
XP requirements are the same for all units (at 10/20/50/100), but the effects for XP vary slightly between units (notably some special powers get buffed, and some endgame units even get new powers). If you want to check them for any unit you can go to the admin interface, create a army->create unit, then fiddle around with its XP values.
After some checking, it appears that the health XP gains very by unit now (+2 per level for weaker units and up to +4 for tankier units).
If you have soldiers in an enemy city then you have the option of taking it. Cities have: A passive +25% defense bonus for all friendly units in it (which can be made much higher through walls). With even a simple palisade, this bonus is enough to turn a slight loss into a clear victory.
Note that this bonus stacks with the defense you get from forests/hills/mountains (+25/25/50%).
Enemy troops also require a full turns movement to enter a hostile city, which means that you have to sit at least a turn outside the walls before attacking. Cities have no passive defenses though, so once enemies manage to kill any defenders, they can take it without any problems).
So a balanced, combined-arms approach to armies is more risky and less reliable than hordes of the same unit? Since, you know, your spearmen might never fight the cavalry; your archers might try instead? And your cavalry impale themselves on pikemen?
Not riskier, but in general sub-par.
There are a few basic unit types depending on attack range and move speed (eg. Infantry, Ranged, Calvary). You generally want to keep with the same kind of unit in a battle (unless there is good reason to use others) so that all your units arrive in the fight at the same time and all the enemies forces have a pool of all your units to choose from when you attack (rather then a few to focus kill at a time).
That said, if you only get a horse per turn, you are going to have to make the rest of your army out of other things.