Thereunto: These maps are an advanced player playground,
I definitely agree with this.
Thereunto: Undead:
1) With plenty of ruined cities on the map, reanimate ruins (
AR) is a key tactical spell. Undead have a unique limitation that they lack the night vision required to cast the spell from large distances. There is a 12 hex range on AR, but you cannot cast it on cities that you have not seen before (and specifically hasn't been undiscovered by trail of darkness, etc).
I mostly didn't have much trouble expanding vision over the earth walls to the ruined cities, since Dark Elf recruits were easily available.
I'm also not convinced all those ruined cities are really that valuable a resource. I had access to Animate Ruins both times, and neither time did reviving those cities seem like the most cost effective use of my limited casting points. They simply haven't mattered in either game, especially because you need to get a digger out there to unearth them.
Thereunto: Highmen:
1) Have access to the most powerful tactical item (Vision IV + Nightvision), which allows them to see threats in advance, despite having a weaker naval presence. Innate true-sight also allows for better early game intel and spotting of enemy leaders.
This is a big thing on a map that limits vision so heavily, and the True Seeing High Men make extra strong use of it on a map that favors so much concealment.
Thereunto: The centre would still be contested territory for more gold and mana, but it can't be used to lock out other players from exploring the other levels of the map.
I'm not sure the center can lock people out as much as you think. I've held it against all comers in two games now, but that's pretty much only because only one person would come at a time. If more than one player attacked the middle, the person in the middle can't reasonably hold it because of how long it takes reinforcements to arrive.
Thereunto: 5) Consider making the centre spells more useful to early game tactics. It's tricky to find a good balance for this. Spell ward, watcher, disjunction, towngate, low tier summons (frog), and Flood (works on the surface only) are some ideas that come to mind.
This is indeed tricky to figure out, since adding the really useful tools weakens the players that have them naturally.
I like the symmetry of having all the [X] Mastery spells available there, but I might be biased since it was my idea.
Thereunto: 7) Never leave a spot on the map that is a single hex surrounded by only impassible rock in the underground or depths. There are ways to get your leader into that spot and sit there forever.
Speaking of which: double check the alters, and decide whether you want other players to be able to claim them. I managed to claim the Azrac altar, and I had a plan for claiming the Dark Elf altar when I had the time to get over there and implement it. I didn't evaluate the Elf or High Man altar, so I can't comment on those.
Thereunto: 3) It becomes possible with long ranged attacks to micro damage indie stacks by shooting and then leaving battle (wash and repeat),
if you have two or more hexes that you can attack from. If you limit access to important locations by only allowing 1 hex entry, you make it more difficult to micro the stack.
This is also a somewhat important note, and it seriously helped my conquest of the surface in both games. It is entirely possible to do with a single stack, but you need something that can pull the bad guys into range of your waiting artillery (I think I might have used a hasted ballista first game, and just attacked from multiple hexes later when I could).
Thereunto: 4) If two indie stacks are touching, race relations can sometimes be used to force the stacks to fight each other.
This is also important. There are a few locations where I claimed something by simply having one set of indies kill the other.