So if you take a castle in Strategus, players have found that the population takes a looong time to come back. In the mean time, this nerfs the gold production per hour by the castle and what is available to people there working is also reduced.
Converting population to troops was removed for understandable reasons, as it led to ridiculously huge armies and gave other clans no chance to compete at production level. But the other way around, should be brought back in.
The biggest example is cities. No one has sieged a city/town/whatever you wanna call it yet. With the current system, you'd kill 15-20k population just trying to take it, most likely the entire population. Cities are a clan's main source of gold generation by players set to work there. Once you kill all the population, gold income per hour will be nearly 0, and if these cities regenerate troops just as slowly as fiefs do, someone calculated it would take nearly 2 years(!!) to get back to 20k. This would be remedied and balanced with adding troops to population back, and perhaps increasing the generation rate per hour for population (since you couldn't get those troops back out in case of a war.)