So I played a magic heavy game on normal with a caster sovereign.  I lost two battles early where I was just over matched by wildlife. I lost one late game where I underestimated Kraxis and sent a second-tier hero and army after his capital.  Otherwise I steam rolled with caster heroes surrounded by mage troops and 2 or 3 front line soldiers in chain armor with the flaming battleaxes.  I ultimately won via the Spell of Making. 

A few things about the leadup to casting:

• I had a relatively large and healthy empire so once I reached the tech requirement for the spell, the towers, and the Forge of the Overlord I went into defensive mode, put all my best troops near the borders of enemy lands, and stuck my Sovereign and his army in my heavily fortified, fort/guardian statue capital.  I started building the towers and the Forge and realized that I could rush them just like any other production.  
• I cranked the tax rate to oppressive, bringing my per turn gold income to close to 600g.  That meant that every 5 to 6 turns I could rush production on one of the four keys to the spell of making.  Even though you're required to build 4 large city improvements I went from researching the necessary technology to buildings complete in ~25 turns.  
• I received no acknowledgement of my efforts from other kingdoms except for one comment from Capitar about the Forge of the Overlord being buried but granting ultimate power if found.  Funny, since I was building it in my capital.  

Here's a view of my income trajectory once I decided to rush production on all four components.  

As for the actual spell casting:

• It immobilizes your Sovereign.  Would have been nice to get some sort of warning to that effect.  I expected it, but the game gave no indication it was going to happen.
• No animation, no particle effects, nothing.  Kinda ho hum for the "I'm about to grab hold of the ultimate power in the world" spell.
• Baffling that no one else in the world seemed to care.  I spent a good 15 or 20 turns prior to reseraching the spell preparing my kingdom for all out war.  Start casting the spell and... nothing.  Can't the other channelers feel that I'm in the process of cutting them off from the source of their power? 
• 500 mana, 25 turns.  Neither seem big enough.  By the time I cast the spell I was bringing in nearly 60 mana per turn.  And that was with my troop producing cities enchanted to the gills as well as several heroes being well buffed.  I was probably pulling in ~80 mana per turn total.  I had just finished two big wars relying heavily on magic, including the more expensive strategic defensive spells like Pillar of Flame and I was still sitting at ~4000 mana.  
• Even if the other kingdoms had all decided to respond 25 turns isn't nearly long enough.  I was well fortified and dug in.  My capital was coincidentally in the SW corner of the map so it was quite a hall from my borders with many natural fortification (mountains, valleys filled with well defended cities, etc) that the opposition would have to go through to even try to attack my capital.  
• The progress bar on the kingdom report screen should have the turn numbers on it as well or should display them on mouse over.  If you're not paying attention to the turn you start casting the spell (I wasn't) you're left to estimate how many turns you have left. 

Overall, I had fun with the game.  But Act III still needs the most work.  I shouldn't be able to go from researching a game ending tech to winning in ~50 turns without any reaction from the opposition.  The towers and forge should not be rushable, or if they are they should be so expensive as to not be realistically plausible.  The final spell needs some visual oomph.  I expected to see little streams of light sputtering from my Sovereign to all the shards that got stronger and thicker the longer the spell cast lasted.  I expected everyone, even my allies, to throw everything they had at me to try and disrupt the casting.  Or I expected them to try to raze or capture a city with one of the 4 components.  (A semi-aside:  there should be a spell of making diplomacy option where you can alert allies that you are going to cast it and get them to sit out the all out war in exchange for serious diplomatic concessions).  

I'm going to play another and try the quest ending.  Conquest and Alliance endings are relatively straight forward, though I would expect opposition to gang up on you as you get close to those as well.  The existence of quest and spell endings (and the systems that make such things possible) are what should set this game apart from more standard TBS games.  Those endings should be spectacular.  The idea is right, but the mechanics need serious balance.