Empire:
I have also been reading through Empire. I have paid most attention to the rulership system. As I mentioned in another thread here on the same topic, it has good points and bad points.
On the good side, it scales well. The design is predicated on there being three scales: Barony, the smallest, where the rulers wield influence over a small region; Kingdom, the middle, where the rulers wield influence over a larger region (theoretically composed of multiple Baronies); and Empire, the largest, where the ruler(s) are overlord(s) of a union of two or more otherwise-independent Kingdom-sized realms. Rather than let the numbers of things grow huge (and unmanageable), the numbers remain small, but the meanings of the "units" become larger.
As far as managing domains, Empire feels like a mix of the Sid Meier's Civilization and the Warcraft style games. The resources are much the same as in Warcraft (primarily Food, Stone, Lumber, Gold, and Iron), and are harvested by assigning a "population unit" to work a "land unit" (a la Warcraft). Settlements start small, can be increased in size, and can have upgrades added to them (a la Sid's Civilization). Because these are all abstract units, they scale with the scope of the campaign.
The 11 main classes are each discussed in terms of how they rule, and 4 additional class features that only pertain when ruling a domain are added to each. This gives the classes good integration with the rules presented, and shows how each can be an effective ruler.
The mass combat system has some good points; notably the inclusion of a "size factor" for units, in which one can choose to create units with variable numbers of creatures (most other systems assume a constant number of creatures per unit). Otherwise, the system is much like many other d20 mass combat systems: it treats a unit in terms of the stats of a single creature.
On the downside, it feels like the book was cut down. All power is discussed in terms of land (merged with the military), with no power accorded to religious, magical, or trade -based organizations. While the mining of precious metals is covered, as well as their use in making trade goods (magic items), there is no mention of gems (for example). Some trade in exotic creatures would be plausible, but is not mentioned. Likewise, the city upgrades are a list that begs to be expanded; I can envision a Shipwright's Guild that produces ships to be sold, used to enable trade, or made part of the navy (a Port upgrade is available, but its function only affects the ability to trade).
Hope that helps.
