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Well, if you have all 7 creatures from different Castles, and even undead, then even expert leadership won't help. Moreover, such an army cannot be replenished - unless you capture a similar Castle. And creatures of level 6-7 almost never join. Also, neutral creatures often require astronomical sums to join. The question is - by the time you develop expert diplomacy and find creatures willing to join, enough time will pass for the AI to develop well and, if not destroy you, then at least significantly weaken your mixed army. Of course, on the "pawn" (easy) difficulty level, every other monster will join without diplomacy. But I prefer to play on "rook" (hard).
1. The difficulty level of the map does not affect the joining of monsters.
2. 90% of players on this forum play on the "queen (or king?)" - "very hard" level.
3. Monsters of level 6-7 join just like monsters of level 1-5, if your hero's army is strong enough.
4. Expert diplomacy (if it is the goal) comes no later than the fifth level (and sometimes even on the fourth) - for this you need to take the appropriate hero-diplomats. And during this time, the AI will not develop - it's only a few days. Again, if joining units during the game is the goal.
So, diplomacy, if it is necessary, is a very useful skill. And if there is an opportunity to play with native units, then it is not needed at all... But on the other hand, if you remember the last Warlords tournament, a diplomat was able to join 2k gargoyles, 700 liches, and more than a thousand vampires - it would take a long time to gather such an army in your own cities :) ...
