As a GM, I'd handle it a bit differently.
Character hires and discusses plans with master mason. The player draws the plans (if he wishes) and discusses it with GM. Mason estimates construction time and costs, which are based on several factors:
1. Quarrying: Not sure how a quarry location would be determined; can stone be obtained anywhere? A quarryman can cut a certain volume of stone blocks per day (I don't know what this is, but 10 cu. ft. sounds reasonable; research needed). Stone's density is about 2.65 times that of water (i.e., 2,650 kg per cubic meter = 165.3 lb. per cu. ft.). (Is quarry-labour subject to the law of diminishing returns, or does output scale linearly with manpower?) One or more masons (or apprentices) would have to be on hand at the quarry to direct the quarrymen's efforts (e.g., the size and shape of blocks).
2. Transportation of stone: Assuming land transport moves 20 miles/day, calculate number of round trips per day between quarry and build site as 20 miles divided by round trip distance in miles (e.g., if distance is 2 miles, a team will make 5 trips per day). Also, a team's carrying capacity needs to be known (e.g., a wagon team carrying 1 ton = 5 trips/day = 5 tons of stone/day per team). Costs of horses, oxen, wagons must be tallied, plus daily expenses. Only 1 man is needed per team, unless guards are desired. Other options: river barge, sledge. A sledge may be preferable to a wagon, or a cart, as it is cheaper, can probably haul more stone per trip, and may not require as many labourers to unload at build site. Assume a team of labourers to load stone at the quarry and perhaps another to unload it at the build site.
3. Construction: Based on the needed volume of stone as calculated from the plans, the number of masons, and the master mason's skill, GM calculates how much can be built per day and how long construction will take overall. Research would be needed, but 20 cu. ft. per mason per day could be reasonable. There would be diminishing returns, as more than a certain number of masons would be unable to work simultaneously. For more realism, the construction rate could be reduced for storeys above ground level--e.g., ground level rate = r, 1st storey rate = 1.5r, 2nd storey rate = 2r, etc.
Of these 3 factors, the GM calculates the one that is the "bottleneck" (i.e., limitations based on budget, availability of labour, etc.); the bottleneck sets the pace for the overall project (e.g., if volume x of stone is quarried/day, there's no point in having a greater transportation and construction capacity).
Once daily costs for labour are calculated, the cost of the castle is simply a factor of time. Construction might slow during inclement weather. Of course, there are other details such as wooden construction for floors, doors, etc., but stone quarrying, transport, and construction would be the main concern and comprise the greatest cost in time and money. Terrain considerations might also apply. Other personnel would be needed, such as overseers, carpenters, blacksmiths, guards, etc. Payment could be negotiated in terms of land, accomodations, victuals, etc., rather than exclusively in coin.
Subject: Castle-building 
Tue Sep 14, 2010 3:15 pm
