This is one of the biggest updates yet. A lot of changes have been made in a lot of different areas. A bunch of inconsistencies and other issues have been dealt with. Keeping an exact log of all the changes is simply too time consuming, but a few tidbits are written below.
Also, in the zip-file I've included an add-on that integrates the units from Steel & Thunder Unit Expansion by Deliverator.
-- TECH TREE
Techs in Eras ahead are now twice as expensive +40% (was +20%). Techs in Eras behind are the same, -20%.
Embarked Units can now ignore cost of ocean tiles with Colonialism. (was Square rigging tech in 2.7.3).
Basically you're encouraged to use naval units to escort your embarked units across Oceans, until you get Colonialism.
Inspired by Angryr's Harsh Existence mod:
Cities no longer start with a free amenity.
Cities without fresh water start with 1 housing
Cities on coast start with 2 housing
Cities adjacent to River/Lake start with 3 housing
Aqueducts give 4 housing (was 2)
Campus now only gets a single +1 science from adjacent Reef(s) until Biology.
Encampment, Holy Site and Theater Square now offer base yields when built.
All Walls now cost 1 gold in maintenance for each level.
HP and Outer Defense Strength from Walls have been changed.
Lighthouse, Shipyard and Seaport bonuses have been changed so that they boost yields of Fishing Boats and Fisheries.
Copper can no longer be harvested.
Harvesting Stone now requires Chemistry.
Various changes to frequency and placement rules.
Extra niter accumulation from Industrialization has been moved to Rifling.
Added info about all improvement yield changes tied to techs/civics where that info was missing (mostly base game/vanilla stuff).
Many improvements now receive Gold from being adjacent to certain Districts.
Many improvements can no longer be built on or adjacent to Snow or Ice.
Fishing Boats and Fisheries now require Harbor district buildings in order to boost their yields.
Forts and other defensive improvements now provide housing, and some small yields.
Fort and Roman Fort improvements can now be placed on all passable features that can't be removed, like floodplains and natural wonders (though the latter seems not to be working). Forts can also be placed on resources which lets you acquire the resource.
Outback Stations yields have been boosted.
Fixed a lot of issues with building improvements on resources with various features.
Inspired by Angryr's Harsh Existence mod:
Barbarian tech level is now 75% of player's (was 50%)
Barbarian kills now give 2 XP after lvl 1 (was 1)
-- UNIT MAINTENANCE AND ATTRITION
Gold Maintenance costs are now increased for all units, scaling up with the Eras.
No extra maintenance costs from having units in enemy territory or embarked.
Instead, Combat units that are outside and not adjacent to owner's borders take 1 attrition damage per turn, unless the unit is inside a district or fort improvement.
And Combat units inside enemy territory and not adjacent to owner's borders take an additional 1 attrition damage per turn, unless the unit is inside a district or fort improvement. Recon units are excluded from this additional attrition.
-- UNITS IN GENERAL
Starting units for each era have been tweaked.
XP amount needed for each level is now multiplied by 3 (was 5).
Various changes to XP earned by units.
Flanking CS bonus reduced to +3 (was +4 in previous version and +2 in vanilla).
Reduced the costs of Iron and Horses in early eras to compensate for the lower income. For e.g. Swordsman now costs 5 Iron (was 20).
Reduced production and resource costs for many units of all eras, particularly the first and last couple of eras.
All classical and medieval units on land now require Barracks or Stable.
All Renaissance units on land now requires Armory.
Naval Raiders no longer have limitations on which units they can escort and help ignore ocean cost.
-- SPECIFIC UNITS
Crouching Tiger, Hwacha, Field Cannon, Machine Gun have all been converted into Support units. They grant escorting non-ranged units a ranged attack and/or a melee bonus, as well as a bonus to City Strikes if garrisoned.
All melee, anti-cavalry, light cavalry, heavy cavalry units and warrior monks now start with a ranged attack of 1 CS, and range 0. This allows these units to make ranged attacks if they acquire one of the ranged support units above.
AT-crew and Modern AT have also been converted into Support units. They grant escorting units a melee bonus vs. Tanks, Modern Armor, Mechanized Infantry and so on.
All recon units may now move after attacking.
All light cavalry units may now pillage for only 1 movement point.
Slinger is now a recon class unit, not as good at exploring as Scout but could be useful in early wars.
Military Engineer moved to Construction (with Fort)
Frigates now require Shipyard.
Ranger moved to Ballistics
Increased combat strength of Tanks and Helicopters
Reduced Aluminum cost for Air Units and Helicopters by 50% or more.
Rocket Artillery and Mobile SAM now cost Iron instead of Aluminum.
Spec Ops moved to Cold War civic (was Lasers).
Observation Balloons and Drones no longer increase the range of Bombard and older Siege units, but now they also increase the range of adjacent Battleships and Missile Corvettes (Cruiser), though the Civ needs to have an oil surplus.
City's now get +1 CS from every district (was 0 in 2.7.3, and +2 in vanilla).
Battering Ram and Siege Tower now grant the same bonus (full damage vs walls) and both work against Medieval Walls. They also grant combat bonuses to their escorting units.
Steel technology now grants enemies full damage vs your cities Outer defenses, but your cities with full loyalty get +4 CS and the Capital gets +6 CS regardless.
Recon units Camouflage promotion now takes the place of "Sentry" at level 2, meaning your Recon units can get stealthy quicker, and other changes.
Military Engineers now complete 20% of buildings/districts production when using a charge. (was 10%).
Spies should now no longer become visible to players on your team, which should include emergency allies (?).
Removed text from all governments about the science and culture bonuses from trade routes. This info should ideally be moved to a Civilopedia section when I've got time.
-- POLICY CARDS
Conscription and Levée en masse reduces maintenance costs but also reduce combat strength by a tiny amount.
Pressure from trade routes reduced to 0.5 for origin and destination.
Military Aid requests are now considered hostile (which should stop enemies from being able to participate?)
Golden Age dedication: "Reform the coinage" only gives +4 gold per speciality district (was +5).
Free Cities now have a base loyalty of 15 (was 10).
Conquered cities now suffer a loyalty penalty multiplied by 75% of grievances (was 25%).
War weariness from combat in enemy lands increased to +3 (was +2).
And much more...
Agricultural Revolution Expanded (AREX) 2.8.6
Tech tree changes, naval movement increased, aiming for historical "accuracy"