In a broad perspective, I'd say that the game suffer from two major problems regarding this issue:
*the way recruitment works
*the linear design of the game
The problem with recruitment is that the players hoard units. If you recruit elite units these are always around, potentially for hundreds of years, if you have the cash to pay the upkeep. This creates a situation where everyone have large standing armies in a time in history when few had more than a handful of troops permanently armed. A better and more realistic solution would be to create a "draft" instead, where unit recruitment might be cheaper but upkeep is horrendous, and of course elites are limited.
The linear design of the game means that everyone start off piss weak and work themselves up to a sort of permanent glory. Once a citadel always a citadel, with the units and income that comes with that. That doesn't create a long-term interesting game but rather a "try-to-win-as-fast-as-you-can" game where by each turn the game becomes easier. A more dynamic design where the realms actually rise and fall would create a longer and more fun game.
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