My favorite Kingdoms build (so far) has to be the Keep. The Keep will ordinarily take four or five turns to reach, but it is mostly cool when you get into the red phase (turns 7-10).
The Keep allows you to count standard units as core. What I really like about this is the variability it gives me when building an army. One could theoretically over-power an army this way, but I find I still buy some core units, just not enough to meet minimum requirements. I like this build, not because it empowers me to make an awesomer

army, but because it allows me to build a
different kind of army.
I have just built the Mercenary Camp and this allows me to use the mercenary faction as my main faction. I have just built a (hopefully) effective HE army and I recruited some core, but less than regular guidelines allow. The bonus for me is I get to try a different faction in a Kingdoms campaign with out having to wait for a whole new campaign. Obviously, we are playing with the optional rules that allow you to choose which faction is to be your mercenary faction. James and I have played three turns with mercs on the board and we haven't found any unbalancing or otherwise unfair/inappropriate combos (besides the
hilarity improbability of an Orcs/High Elf alliance.

Certainly nothing as devastating as Andrew's stand and shoot Undead army.
