The only tech tree layout nit I would pick would be that I would have swapped the Positions of "Warships" and "Navigation", and I would have had Navigation retain the Frigate, and would have placed the SOTL with the Warships tech. So essentially keep the functions of the techs pretty much as you have them in the places you have them, just swap the tech names and icons.
So under my "tech tree theory" you have the nastier more powerful brute-force melee-combat Age-of-Sail "Warship " requiring specifically the "Warship" tech. The less powerful but ranged-attack Frigate is available earlier. I would also keep the Seaport with Navigation even when swapping around so that the basic layout re the top of the EE is
Code:
Navigation ___________> Warships __________> Armor Plating
|_____> Piracy __>__|
And the unit availabilities would be:
Code:
Frigate _________> SOTL ________________> Cruiser
|_____> Privateer __>__|
This also gives more seperation between the Frigate and the Cruiser, and makes building a Frigate a more viable choice to make. The SOTL still has pretty good seperation in this design between where Warships would become available and the upgrade Ironclads at Steam Power.
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Possible code-bug -- did you really intend the SOTL unit to become obsolete at Electronics? This is an entire era later than when the Ironclad (to which SOTL upgrades) becomes available.
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Even though I intend to agree with Natan's basic sentiments from a hysterical historical point of view, I can't see any good alternatives for a redesign that aren't a massive start-over overhaul. Sure, Armor Plating should require Manufacturing or Industrialization as well as Steam Power in my opinion to make a better fit to historical sense, but this makes redesigning the whole concept and the whole tech tree necessary in my view to very little actual gameplay advantage. In the end it has to be what makes sense from a gameplay and gamebalance point of view always trumping historical accuracy. Firaxis themselves has done so multiple places in the standard tech tree. Witness thereof, Industrialization as a prerequisite to Steam Power when it ought really to be the other way around (the mines of Scotland at the time were only able to run as deep and be as productive because of the steam engines pumping out the water in the shafts and powering the air circulation systems, and the productive output of these mines were as much in my view responsible for the industrial revolution in the UK as was the adaptation of watermilling to the powering of factories)
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[edit]My post was in response to #s 210/211-ish but got ninja'd be several intermediary posts that occured while I was thinking, writing, previewing, fact and code-checking