The next topic would seem to be the improvement of land, or tiles. Grassland needs farms, forests need sawmills, hills need mines and fishing grounds need boats. How is it done?
We cannot hope to cram every feature from Civilization IV, or C4, into the D&D system. The video game solves the problem by creation a worker (or a fishing boat to a much lesser degree) which then improves the tile endlessly, limited only by how much time it takes before it can move on to the next task. There is no practical reflection of the "worker" in D&D, nor should there be. Every villager is a worker. The "worker" in the video game is merely a tool to abstractly represent the villagers at work, and we don't have to include the tool in this system.
The solution is simply to translate the improvements succeeded by the worker into hammers ... recognizing that unlike the worker's inexaustibility in Civ IV, the improvements will cost new effort every year.
In C4, to build a worker it requires 60 hammers. Conversely, to build 8 farms on the surrounding 8 tiles, it will require 40 turns (we can dispense with movement costs). If we include serfdom into the calculations (workers build 50% faster), I think it is 3 turns per farm, which would mean 24 turns.
A village with one hammer would then require 60 turns to create a worker, and another 24 turns for that worker to make all the improvements. Therefore, we can simply argue that a village with 1 hammer would take 84 turns to make all improvements (sans worker). 2 hammers, then, could take 42 turns and 3 hammers 28 turns. To improve 1 tile, then, should take 1 hammer 10.5 turns ... but lets be generous and say that 1 hammer produces 1/10th the work necessary to one farm - or, in effect, 1 hammer does the work of 1/2 of a worker's turn in C4. The cost of a "turn" is therefore 2 hammers.
Therefore, if a road needs 2 turns, that requires 4 hammers - 4 years maximum, or less time if the village has more resources (the "road" would be a trading class road, and would reach from the village into one of the adjoining tiles - or one tile to another). A farm would require 10 hammers. Cutting down a forest would require 6 hammers.
Notably, to take advantage of fishing grounds would cost what it is in the video game: 30 hammers.
Now, I would tend to agree that the masters of a territory are entitled to the bonus in hammers that comes from cutting down a forest. The video game calls this, I think, 20 hammers for the quick game. I'm not certain, it may be 30 hammers for the epic game ... I'd have to check it. In either case, it represents a rapid outpouring of work, drawing labor (presumably) from all around, enabling a player to rapidly develop multiple tiles around his or her village. Almost certainly the destruction of a forest would be the first step in developing an area ... if there is a forest, naturally.
The question will arise among players: if I don't have enough hammers, and I'm ready to go elsewhere and pay for more, can I have them?
The answer must be NO. You must conceive that everyone, everywhere in the world is already making maximum use of their hammers to make the stuff that they desire for their territories. In C4, it would be the equivalent of asking the Romans to lend you, the English, a few hammers to help you build the Granary you want. It just doesn't happen. Additionally, even if the party were to bring in hundreds of laborers whom they seized or paid from elsewhere, this would NOT increase the number of hammers, since having a lot of workers doesn't increase the necessary resources or skills. Arguably, a thousand strangers could cause many skilled workers to seek lives elsewhere. On the other hand, however, a thousand new "settlers" could tip the balance into upgrading your village to where it could work three tiles instead of two - that could increase your available hammers. And there is the slavery rule ... use up your population to finish things that are important to you. I can't quite give numbers on how many population you would consume to make a granary - I rarely play the slavery option when I play C4, so I'm not overly familiar with it. A conscientious game designer should be able to work it out.
I don't have the kind of players who would use it anyway.
Other civics increase hammers, however, as do some city improvements. Culture, if it is included, could still widen the available environs and permit forest destruction. I would tend to keep with the C4 rule that destroying forests outside your control does not bring extra hammers ... the important thing is the industry created by the village being near all that activity. If the activity were far away (out of cultural influence) I do not think the village would benefit.
So you have your party taking over a piece of completely untouched land, say 8 forest tiles, two of them with hills, and no development whatsoever. If it is a forest on plains, each tile has two hammers ... but if it is a forest on grassland, it is only one hammer per tile.
This would mean that the starting party has 2 hammers altogether, counting the village and the one tile the village can work. To chop down the forest and bring those extra hammers to work would take three years. It would be nice if the party didn't have to wait even one year - though technically, there's nothing to stop a party from all declaring that they are three years older and so start spending their gotten hammers.
Imagine: say the forest produces 6 hammers per year in the time that it is being cut down (we don't have to strictly adhere to C4's rules - we can reckon everything in 1 year periods). The players could then use those 6 hammers, plus their usual 2, of those gained hammers to finish chopping down the first forest the NEXT year, plus 2/3rds of another one ... which would yield 27 hammers. During the third year, the party could finish the 2nd forest (with the 2 hammers they usually receive), plus 4 more forest tiles ... with three hammers left over to cut down half of a 5th. For the 4th year, then, the party would start with 99 hammers. There'd only be 1 and a half forests left, and the remaining country would be moved over into farms (with all those hammers, it would be easy). Of course, there wouldn't be many hammers left over once the bonanza finished. An environmental approach is slower, but more beneficial in the long run.
If it seems that the time frame is completely unreasonable for a party who adventures and runs around every day killing things, you're right. The party would have to accept that encounters were something that happened only every few months, enabling them to enjoy the expansion of their kingdom. As a DM, if you make your party understand that it can take months to, say, create an army to attack the lands next door, you might be able to have that time pass for them without their becoming resentful. Campaigns, too, take a long time, since armies move so very slowly. In any case, now then have a reason to conquer - taking over someone else's lands and how their hammers are spent becomes an interesting matter.
However, now I'm at the point where I have to discuss the creation of units - something to which I haven't been looking forward. At least I'll have a little longer to piece it all together.