Eden Studios have created a truly impressive pen & paper resource management sim. They describe in detail how a DM and players can incorporate these rules into their D&D 3.5 campaign. For 10th-20th level characters who decide to establish a stronghold and raise an army, this book provides a comprehensive, coherent basis for adding wargaming to your roleplaying.
However, there are aspects of these rules that are unsuitable for my own game: the rules operate on a fixed level of detail and impose certain assumptions on the way that settlements function.
Mass combat involves units of 100 basic troops, or their equivalent (a handful of giants, or one dragon). Resource management employs hexes with maximal diameter of 12 miles (just under 100 square miles in area). Since the empires in my game span millions of square miles, it is infeasible for me to describe every hamlet and outpost in such painstaking detail. Likewise, clashes between armies of thousands would be very time consuming to simulate on that scale.
The way that settlements work in Fields of Blood seems quite different to what I would expect from the Dungeon Master's Guide and DMGII. Rather than extrapolating from the D&D rules, there are certain abstractions and changes that do not sit well with me as DM.
The first of these is population growth. By investing resources in a specific settlement, the player can "upgrade" it from hamlet to village, from village to town, and so forth. The rationale for this is obvious: even Elves would be unlikely to establish a settlement and see it grow into a bustling city within their lifetime. However, although it makes the game more interesting, this isn't a rule that I would want to incorporate into my campaign world.
Another issue that I have is with the role of clerics and wizards in society. In Fields of Blood, each settlement has a single dominant religion, with an associated place of worship. None of the other myriad smaller cults that might exist have any impact on the game. This is in complete contrast with any d20 city sourcebook I have read. One might view it as simply an abstraction, but to me it doesn't fit with the standard D&D fantasy setting.
The existence, power level and political involvement of a Mage Guild is a tricky thing to establish within the 3.5 rules. Within a settlement, the level of any sorcerers or wizards is lower than most other classes. The Fields of Blood explanation is that most high level arcane spellcasters live in ivory towers, preferring to isolate themselves from society. The solitary archmage is a common trope of the fantasy milieu, to be sure, but you need to decide whether this fits with your conception of 3rd edition D&D.
Overall I liked this book a lot, but can't see how I could make use of it without redesigning my campaign from the ground up.