Anti-magic forcefields sound cool as a one-per city building unlocked in high-end magical tech tree. Building such a building, would cause a Magical Nullification shield to envelope the city.

Good target for Sabotage via Assasins/ Saboteurs

Yep, we need to have a protection magic/counterspell system like this, else magic will be way too powerful. We've talked about this already btw.

I like the idea of powerful spells taking a while to cast and everyone knowing where they're targeted.  I think Master of Magic had something like this for global affect spells.  

If someone is trying to cast a volcano spell then it would take maybe 10 turns to complete.  During that casting time not only is your Sovereign unable to do anything else, but a nice glowing/pulsing circle appears on the tile the spell is targeting, giving the target time to cast a counterspell.  In the case of spells that have a global affect, a simple notification to all players would suffice.

Later,
LAR 

My opinion on this: I totally don't like the idea of the targeting. It's... not natural (though the word "natural" in reference to magic is funny ), I think that caster should come close anough to the target to cast something big, then if the opponent doesn't see it it's all to blame his intelligence. Or maybe you can get some "searching" spell that will track such spellcastings of your enemies (actually the location of a caster). I would be fine with that, but "targeting mode"... it's just not natural))

Or there can be introduced sabotage. Destroy the building with agents->cast a spell->try to stay alive after creating such a commotion.

Well think of it this way, continuing with the Volcano spell example.  If someone is casting something that's powerful enough to raise a freakin volcano, then there's going to be a massive amount of "magical" energy focused on that one point.  Being attuned to magical energy your Sovereign, and maybe some heroes, would be able to notice it.  Maybe even limit the ability to detect "targeted" spells to the immediate area around your Sovereign and magic wielding heroes.

Instead of having the game be all "Hey someones definitely forming a volcano under your city" it could be more vague.  You'd be able to see that someone's doing something at a given location, but without further investigation, casting an identification spell or something, you just don't know what it is.

Later,
LAR 

Fardeil:

Unfortunately, 1 turn=1 year.  This makes it a bit hard to link it to astral things, as a year CAN be a rather long period.  Unless this is like a brown dwarf star or something and so a earth-like planet was really close and a year is a couple of days...

Hmmm....

I know your Sovereign ages slowly, and I'm pretty sure he won't die of old age, but I'm not 100% sure about that last one.

Later,
LAR 

I think a short turn delay on casting spells big enough to destroy a city coupled with having to cast counter spells manually is the way to go.  Building "anti magic building" doesn't sound good, nor add to strategies revolving around slinging spells across the map.

There should not be any way to auto counter spells (buildings or otherwise).  Countering should be able to be done through either a dedicated counterspell spell, or casting the opposite spell.

eg:   using the example above, Sov 1 starts casting volcano on your city.  Turn delay 2 turns (i said short!!).  If Sov 2 decides he wants to counterspell, he needs to the mana and a spell that will counter the volcano spell.  So, he can cast a "Counterspell"  or, he might have lower land, he could cast that as the counterspell to Sov 1's volcano.

In my belief the general counterspell should be more costly than a specific counterspell.  For example, "counterspell, might cost 50 mana, while "counter raise land spell" should only be 30 or so.

End note:  from playing the game right now, some (i haven't used all of them) spells have a max range that they can be cast.  The max range is fairly short too (like 4 squares).

If you can move and cast and have it all resolve in one turn, then my Sov would be able to come from outside your LOS close enough to a city, and cast volcano wrath before yuo have a chance to do anything about it.   all you need is movement +4

Perhaps something like this...

1. Each city has some sort of structure that promotes a degree of magical defense. Sacred sites, temples, elemental crystals, monuments, etc. all contribute to it, some more than others. For example, a FIRE crystal node will offer some resistance against AIR and EARTH magic, but will offer more resistance against WATER magic.
2. Structures initially give 100% of their bonus (IE, if a temple gives me 5 resistance, the first one always gives 5) and each subsequent structure of the same nature offers 10% less. Thus, building 10 temples will offer me 5+5+4+4+3+3+2+2+1+1 = 32.
3. Combining certain structures offers either higher benefits or lesser benefits. Thus, building a temple on a holy site will give a fairly large % boost to what is offered by both. Maybe an elemental altar or somesuch build next to/on top of an elemental crystal will offer the same % boost.
4. Resistance either acts to totally negate a spell (at 50% resistance, there is a 50% chance that a spell focused on a certain tile within protection will utterly fail) or to partially negate a spell (at 50% resistance, there is a 50% reduction to magical effects within protection).
5. A Sovereign or strongly imbued champion can enter into some sort of protective meditation, temporarily loosing all of their mana and essence (spreading their energies in the area around them) and being incapable of movement or action for the whole of a turn. The following turn, the Sovereign or champion cannot do anything still, but their protection has faded. Basically: turn one, the Sovereign takes all of his/her energy and places it around him/her. Turn two, the Sovereign withdraws that energy back into him/herself and become in touch with the world again, remembering that the Sovereign has spent the last full year in meditation. This acts to prevent spamming of essence-protection and careful planning.
6. A near-end-game thing could be a spell that prevents magical targetting, or perhaps a structure that prevents magical targetting.

Pros: Using buildings requires a material investment. Buildings can be destroyed, allowing for balance. Reduced benefits from trying to spam buildings, allowing for balance. System can be lore-worked to make sense. The ability to temporarily spread all energies around a selected figure allows for temporary, but vast, protection.

Cons: Economically powerful factions can use high reliance on varied buildings to protect themselves magically. Reliance on buildings can make for some locations to be little more than anti-magic zones.