
Chateau Gaillard 
Previously I looked at some
realworld data for medieval castle construction costs and found, somewhat surprisingly, that if we read the costs from OD&D in silver pieces (groats; 1/3 shilling) that they're quite close to the actual prices involved. I'm still jazzed at how sweet that was!
But one thing I couldn't figure out at the time was a way to estimate castle construction
times. The realworld data was all over the map and not consistent; a small single keep at Peveril could take 2 years; medium to large castles like Orford or Dover took 8 or 10 years; and then on the upper end the "vast" castle of Gaillard which cost twice as much as Dover (and on a difficulttoreach precipice) itself took only a lightningfast 2 years. So I left that puzzle for a later day.
Well, that day is today. The players in my ongoing campaign are advancing in experience and treasure enough that they're starting to ask about options for castleconstruction. The night of this writing I sat down and played with some numbers and discovered a remarkably simple rule that gives fairly realistic results. Here it is:
The base time for construction is the square root of the OD&D total cost, read in weeks. If speedier construction is desired: Each multiplied cost factor divides time by a like amount, up to quadruple cost/speed.
Let's compare that to the realworld data; it's a small sample size, but for gamedesign purposes I'm comfortable making a decision on this basis. Gray and yellow highlights are things added to the spreadsheet since last time. The bottom row for Gaillard is special, because it's the only one where we're applying our quadruple speedup rule.

Castle Construction Time Estimates 
In the 8th column, we have our squartroot estimate for time in weeks. For example, with the top row of Peveril, we take the square root of the D&D cost (identical to real cost in this case): sqrt(12,000) = 110, rounded to nearest whole number. Dividing that by 52 weeks in the year comes out almost exactly to 2 years, exactly the realworld time it took to build it (in the 4th column). Doing that for Orford and Dover likewise comes out within 1 or 2 years of the actual figures.
Now let's look at the last case of Chateau Gaillard. Just looking at a map of the place, it looks smaller than Dover Castle (compare "Details" in the 5th column). My estimate using the OD&D tables it that it should cost about 208,000 sp or so; and construction time ought to be sqrt(208,000)/52 = 456/52 = 8.8 years or so. But to this we will apply a speedup factor of 4, quadrupling both price and speed of construction; then the price jumps to 832,000 sp and the time drops to 2.2 years. Note that these figures now align with the realworld prices: Gaillard cost some 720,000 groats (i.e., 12,000 pounds) and did indeed get built in just 2 years time.
Consider:
Gaillard was the major work of Richard the Lionheart, who worked feverishly to stake the world's finest castle directly in the heart of his French enemies. He personally supervised the work and drove laborers unrelentingly, even through reported rains of blood. Said one observer:
... the king was not moved by this to slacken one whit the pace of work, in which he took such keen pleasure that, unless I am mistaken, even if an angel had descended from heaven to urge its abandonment he would have been roundly cursed.
Note also at least one nifty sideeffect of our square root rule: Designing a large castle up front will take overall less time than if you build a small construction and add to it over time. Say, two separate rounds of 10K construction would take sqrt(10K) * 2 = 100 * 2 = 200 weeks. But one round of 20K value construction would be just sqrt(20K) = 141 weeks. This sort of jives with the classic engineering experience that it's more efficient to get a design right early rather than late; and provides a neat ingame dilemma on whether a PC should get started with something small with available resources, or wait to gain more treasure so as to start on a larger (and ultimately faster) construction.
(N.B.: There's a bolt from the blue waiting for the first person who suggests Agile castle construction methodology.)