I'm basing the following on the idea that palisades: early game (tier 1-2), walls and ring-thing (palisades obsoleted): mid game (tier 2-3), and castle: late game (tier 3-4).
So early game base str is 4.25 (pop 5 city), culture bonus of 25-50% gives us 5.31 - 6.38
Lets call axeman our early game attacker, str 4, lets call him lvl 2 (training yard, apprentice civic, and one of the myriad barbs running around) with combat I and City Assault, final str 5.6. Looks just about right to me there, 2-3 to take your capital city's militia, 1-2 for a satellite city.
If you focused on it you could probably get one of these out by the time that Orthos came calling, but you'd be way behind the other civs as far as turn advantage is concerned. Orthos has a min str of 6.25 (I've never seen orthos with no promotions, but it could happen I suppose) so one might help save your city, but would be unlikely to hold him up by itself.
Mid game we've got two defensive buildings in walls and the ring-thing, so we've got a base of 6.5 (pop 10 city) 40-80% culture, gives us a final str of 9.1 - 11.7.
Lets call our attacker a str 7 maceman (I think they are str 7, please correct me if wrong), combat I-II and City Raider I (you should have plenty of lvl 3's at this point) for a final str of 12.6, or 11.2 if lvl 2 (city raider I and combat I)... looks like one should do the trick to take out your garrison troopy. Granted this is against a unit designed to take cities, but that is what I'd use to take a city.
Compare this to a Lvl 2 Longbowman in the same city, base 6 garrison I-II brings us to 11.4, 40-80% culture brings us to 15.96 - 20.52. This seems about right to me, longbowmen are much more expensive and require a special building to be built.
Now in the late game we've got all three defense buildings and good sized cities ( unless you are playing the vamps...) so lets say pop 20 for a base str of 9, 80% culture bonus for a final str of 16.2... we've almost caught up to the longbow in the earlier city.
All in all it looks like you've designed a unit that is best in the early stages of the game but continues to be useful throughout the entire game. Although it gets eclipsed by more expensive units in the later stages of the game it is still nothing to sneeze at and can stand up to anything short of a hero or tier 4 unit, which I think fits thematically with the idea of a city militia as well. If you put the time and treasure into "training" them (developing the techs to be able to build the buildings) you have a staunch but not overwhelming defensive force.
Uh yeah, did I mention that I really like this idea?
~p
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