Heroes 5.5 WarGame Edition - Tactical Edition.

It is not possible to remove the connection from the Citadel to the Knights, as this would lead to excessively powerful builds based on Knights and Angels. Neither faction needs the Citadel; something more powerful can be built instead.
Griffons are safer in this regard. However, if you remove the connection from the Citadel, it will lead to a situation where, upon reaching level 6, Griffons without the Citadel will be the obvious choice, as they will significantly outperform other options in terms of power.

That's why I prefer the standard 6-tier scheme of 0-3-6-9-12-15; it offers more room for maneuvering when building structures and doesn't compromise the variability dependent on Fortresses.

In the standard scheme, there could formally be more options, but they are not balanced, meaning there are far fewer effective options.
It could have been, because the tiers from 0 to 15 simply kill even formal variability (too slow and linear development).