[Arryn]

In one of the various threads on playing a specific theme (I forget which theme and the poster, may have been Zen), it was recommended that the el-cheapo zero-DP fort type be used so that you can quickly and (relatively) cheaply pop up forts all over your borders both to channelize the enemy and to allow recruitment of national units right at the front. This works best for nations that do not need to maximize resource gathering (ie: whose main limit is gold, not resources).

Other than this particular example that was cited (I do wish I could recall what thread it was), the Castle which Norfleet mentions is pretty much the best all-around buy.