Quote Originally Posted by Didz
In my game you can start re-training units after one turn in a newly captured city so that isn't really an issue.
That all depends on how you capture it, the replenishment rate of the pools, and how the folks you captured it from left it. If you occupy, the existing pools are left untouched, sacking cuts them by 1/3rd and extermination empties them. If the AI has recruited everything in sight before you arrived, then even occupy will leave you with empty pools.

Meaning that if you lengthen the time needed for one new "unit" to appear in the pool, and/or reduce the maximum pool, and couple that with increased public order problems, then even a sacked city can become impossible to hold if you are to move your whole army away next turn, as witnessed in LTC.