phaolo: Is it unnecessary to
convert all captured cities?
They seem to revolt easily otherwise, without many units guarding them.
You should migrate unhappy/rebellious cities, yes. But whether they rebel depends on alignment and racial relations.
Basically, if you're good, cities of other good races will like you, (their neutral cities and units will voluntary join you for gold, instead of fighting you) neutral races will be ambivalent (won't join you but won't rebel either) , evil races will be displeased. (Their cities will rebel, their units will desert unless you put them in big armies of loyal units/use units with bard skills)
If you're neutral, you can make friends with anyone. If you're pure good or pure evil (Undead or High Men) you'll have a lot of trouble since even neutrals are rebellious.
But it's not fixed: you can boost racial relations by building walls or upgrades for cities of that race, or by migrating cities to that race. You can lose relation points by migrating or razing their cities, or by declaring war on players of that race. This can be a nasty surprise: I've had games where my Good aligned Elves build a lot of human units, declare war on a human player, and then the humans in my armies start deserting. Oops...
These shifts to the relations will slowly revert to their base level. But if you do enough nice things for a race, you can certainly be on friendly terms even though you didn't start out that way.
phaolo: Questions: 2- can flying units conquer\raze all buildings\cities? I was surprised when they did so to some towers.
Yes.
But razing cities takes time and they may rebel/be liberated first. And razing strong sites will require several units to raze them. Also, razing buildings will often spawn "avengers" that will try to kill your scouts.
3- who can activate altars? Only heroes, any unit, the owner whenever they want?
Once captured, (i.e. your flag is hanging over them) altars can be used just by clicking on them.
They require a long time to recharge OR a lot of mana. (500 at zero charge, I think.) Since the AI gets ridiculous bonuses to mana and income at high AI levels, they can and will spam altar spells. This can lose you games.
Let's just say that taking out altars is something of a priority for me when facing such AIs, and that I really like the underground. (No altars there.)
4- I don't see any way to offer peace in the relations panel. Just a campaign thing?
You need to select the leader, not the race. (i.e. the top-row pictures.) Then there should be an option to offer peace. But they're unlikely to accept in the campaign.
5- archers don't seem to hurt flying units much, unless I choose the automatic battle. Wtf?
Are you positioning them right? Cover will reduce the archer's chance to hit.
In general, auto-battle has some quirks and abstractions and will always allow defending archers to shoot at least once, even if in tactical battle you can position fast units to charge before the archers can shoot.
6- only battering rams can destroy walls\doors?
And catapults. And Goblin Bombers. And units with "wall crushing," like Goblin Beetles, Dwarf Moles, Frostling Yeti. And spells like Fireball and Great Hail. And boulder-hurling giants.
You can also set wooden walls on fire with flame throwers or the "ignition" ability/spell. However, oddly enough, having units with fire attacks won't let you attack walled cities/towers unless you also have a unit that can damage walls directly.
Likewise, the game lets you capture a dozen towers/walled cities in a row with 1 bomber, even though you'd need 2 or 3 bombers to destroy a single wall.
About next map, should I choose Dwarves or Lizards?
Who is the most useful \ stronger?
Suggestions about the next 2 spheres?
Up to you. Dwarves are a bit stronger, but lizards can swim. They even get swimming artillery, which is pretty awesome.
The story is different for each faction choice you make also, and choosing lizards will give you different options further down the road than choosing Dwarfs.
As for spheres: I'd pick 2 water. High level water spells are very cool, whereas low level air and life spells are better. Specifically, 3 water spheres get you spells like Liquid Form, (great to cast on your leader, halves the physical damage he takes) Great Hail (Massive damage in an area, and crushes walls. Also looks cool) and Healing Showers. (Area-effect healing.) Also Frost Beam (AoE disabling/damage) and Geyser (heavy single target damage.)