Basic thought: build Great Wall, then instigate wars with civs that hate you anyway but have to sail to get to you, then ambush them with siege engines and some decent mop-up crews and reap the double-great-general points from Great Wall.
What started my thinking about the secondary ability of the Great Wall a lot more:
I've all but wrapped up a Monarch game in which I randomly got assigned Asoka (spiritual/organized). My game plan was to found a religion early, spread it around, and while I waited for it to spread, kill at least one neighbor to expand. I'd then get my religious allies to DoW on the infidels and lead the charge for a Domination win. What actually happened was a bit different.
I beat up my Egyptian neighbor early and expanded as fast as I could to block off my rexing neighbor Tokugawa of Japan. I have had bad experiences with Toku before, so I decide to kill him sooner rather than later. I built the GW to seal myself from barbs. The GW would thus allow me to use just warriors in city garrisons and throw the entire weight of my axeman army against Toku. Okay, I admit that another reason why I wanted that GW was because I overexpanded and couldn't afford to simply build a lot more axemen.
Suddenly Toku converts to my faith and actually warms up enough where he'll tech trade, so I decide to let him live and act as a human shield against Napolean, who keeps demanding tribute and getting turned down. I know that France will eventually DoW on me for that, and Spain.. well, I think we all know that if you don't share Spain's religion, it's only a matter of time before Spain declares war on you.
Now this is the weird part: I played Fractal map, and I apparently wound up on a long, snake-like continent where Russia is at the extreme western end, then France a little east of there, then Japan east of there, then India (and the former Egypt) smack in the middle, then a wall of four mountain peaks cutting me off from Spain to my east, and farthest east is Rome. There is one large island to my northeast, but Spain colonized it before I could get there.
Toku is a jerk to everyone except me, so nobody has open borders with him. So when Napolean DoW's me, I shrug it off and expect Nappy to send a couple of galleys of crap my way, and he does, but I am waiting for him with a couple of catapults and axemen. He tries a few more times before I get bored and ask for peace, and he gives me some gold to boot. I should have just kept warring with him and reaping those experience points. Later Spain DoW's on me and runs into the same problem as France: I am waiting for them with a siege and mop up crew.
Later in the game Catherine of Russia DoW on France TWICE; both times she asked me to help, and I agreed both times since Catherine was my strongest competitor (so I don't want to tick her off) and Japan STILL had no open borders with anyone other than me (so France can't do much against my declaring war on them). Japan shamelessly dogpiles on France until France is forced to sue for peace with both of its adjacent enemies. Again, I sue for peace but should have just let France attempt to land something on my shores.
Now, I want to kill Russia as it is my biggest competitor, but I can't directly invade them because that would require my marching through Japan (no problem) and France (major problem because Napolean hates me). So I sigh and decide to wait for tanks and airports so I can ship a big squad of tanks and infantry followed by a trickle of reinforcements via airlifts.
Japan asks for a defensive pact, and I agree, figuring that I needed SOMEONE to be an ally this game. In hindsight I shouldn't have agreed; I should have made myself more of a target. I missed out on lots of potential experience points there, bah.
But then Russia abruptly issues a DoW on me, and Japan takes the brunt of the attack for me. I give Japan some pretty lopsided trades in their favor just to make sure that they won't lose cities to Russia. France surprisingly asks for Open Borders not long after--how convenient; I wonder if the AI is smart enough to realize that it's in the way between 2 superpowers and thus better give open borders or risk getting caught in a crossfire. Or maybe it was the tribute that I gave Napolean out of pity since he was in dead last place? Anyway, I sack 2 Russian cities that were, judging by their names, former barbarian cities that happened to be deep in French territory that Russia captured before France could. France then abruptly retracts its Open Borders, trapping the bulk of my army. Naturally I DoW on France and capture all but one of its cities--Japan dogpiles and takes a holy city before I can get to it, ugh.
Spain and Rome DoW on me soon after right as I am poised to invade the heart of Russia. You can guess what happened. I'm of course dying of laughter by this point at all these failed invasions-by-sea.
Now of course, I have the Great Wall, so all throughout this weird game in which nobody can attack me with land forces directly except my best buddy Toku (and he sure isn't going to attack me, not when we're the only Buddhists in a world full of heathens), I am spawning great generals since you get them double-speed if the combat takes place within your cultural borders. It's just getting ludicrous at this point; I have a dedicated Modern Armor factory that spits out a 17-xp Modern Armor almost every turn (military academy, heroic epic, theocracy, vassalage, barracks, 2 military instructors, West point, Pentagon, and a TON of waterwheels, workshops, farms, and mined hills), for instance. I also have 3 almost-invincible Modern Armor warlords by now, one of which is Combat VI, Drill IV, Morale, and Leadership enabled, with the others lagging a little bit in the Combat promotions department. I have another great general attached to Mech Inf, though that Mech Inf used to be an axeman back in my Egypt-pounding days.
So this is got me thinking that I should try to instigate some early wars with civs that hate my guts anyway, like crazy-religious Izzy. Obviously my situation was pretty sweet in that I was best buds with my buffer zone pal Japan and had a wall of mountain peaks to keep Spain at bay, but man, I can see the Great Wall being very useful on archipelago maps for builder types. With all of the generals I got, I could have just made them into military instructors and cranked out units that all START with 27 xp.
Also, Parthenon and Pacifism might work in conjunction with GG-making, but I'm not sure--anyone know? And an Imperialistic player would have another doubling. So I suppose in theory you could get a +100 GW bonus, +100 Imperialistic, +100 Pacifism, +50 Parthenon = +350% rate of GG making, or in other words, getting them 4.5 times more often than normal. I can imagine a city cranking out units that START with 37 xp.. now that's a scary thought.
(Edited to correct my unfortunate typo of "seige". Also because I wanted to be clear, in case it isn't obvious to any civ veteran, why you want only NAVAL invasions: if you declare war against someone who can invade you via land routes, you stand the risk of them pillaging stuff around your frontier cities. With naval invasion stacks, you can pound away on their one big stack with as much siege as you want followed by a mop-up crew. No pillaging, as the enemy soldiers just step step off the boat and immediately die.)
What started my thinking about the secondary ability of the Great Wall a lot more:
I've all but wrapped up a Monarch game in which I randomly got assigned Asoka (spiritual/organized). My game plan was to found a religion early, spread it around, and while I waited for it to spread, kill at least one neighbor to expand. I'd then get my religious allies to DoW on the infidels and lead the charge for a Domination win. What actually happened was a bit different.
I beat up my Egyptian neighbor early and expanded as fast as I could to block off my rexing neighbor Tokugawa of Japan. I have had bad experiences with Toku before, so I decide to kill him sooner rather than later. I built the GW to seal myself from barbs. The GW would thus allow me to use just warriors in city garrisons and throw the entire weight of my axeman army against Toku. Okay, I admit that another reason why I wanted that GW was because I overexpanded and couldn't afford to simply build a lot more axemen.
Suddenly Toku converts to my faith and actually warms up enough where he'll tech trade, so I decide to let him live and act as a human shield against Napolean, who keeps demanding tribute and getting turned down. I know that France will eventually DoW on me for that, and Spain.. well, I think we all know that if you don't share Spain's religion, it's only a matter of time before Spain declares war on you.
Now this is the weird part: I played Fractal map, and I apparently wound up on a long, snake-like continent where Russia is at the extreme western end, then France a little east of there, then Japan east of there, then India (and the former Egypt) smack in the middle, then a wall of four mountain peaks cutting me off from Spain to my east, and farthest east is Rome. There is one large island to my northeast, but Spain colonized it before I could get there.
Toku is a jerk to everyone except me, so nobody has open borders with him. So when Napolean DoW's me, I shrug it off and expect Nappy to send a couple of galleys of crap my way, and he does, but I am waiting for him with a couple of catapults and axemen. He tries a few more times before I get bored and ask for peace, and he gives me some gold to boot. I should have just kept warring with him and reaping those experience points. Later Spain DoW's on me and runs into the same problem as France: I am waiting for them with a siege and mop up crew.

Later in the game Catherine of Russia DoW on France TWICE; both times she asked me to help, and I agreed both times since Catherine was my strongest competitor (so I don't want to tick her off) and Japan STILL had no open borders with anyone other than me (so France can't do much against my declaring war on them). Japan shamelessly dogpiles on France until France is forced to sue for peace with both of its adjacent enemies. Again, I sue for peace but should have just let France attempt to land something on my shores.
Now, I want to kill Russia as it is my biggest competitor, but I can't directly invade them because that would require my marching through Japan (no problem) and France (major problem because Napolean hates me). So I sigh and decide to wait for tanks and airports so I can ship a big squad of tanks and infantry followed by a trickle of reinforcements via airlifts.
Japan asks for a defensive pact, and I agree, figuring that I needed SOMEONE to be an ally this game. In hindsight I shouldn't have agreed; I should have made myself more of a target. I missed out on lots of potential experience points there, bah.
But then Russia abruptly issues a DoW on me, and Japan takes the brunt of the attack for me. I give Japan some pretty lopsided trades in their favor just to make sure that they won't lose cities to Russia. France surprisingly asks for Open Borders not long after--how convenient; I wonder if the AI is smart enough to realize that it's in the way between 2 superpowers and thus better give open borders or risk getting caught in a crossfire. Or maybe it was the tribute that I gave Napolean out of pity since he was in dead last place? Anyway, I sack 2 Russian cities that were, judging by their names, former barbarian cities that happened to be deep in French territory that Russia captured before France could. France then abruptly retracts its Open Borders, trapping the bulk of my army. Naturally I DoW on France and capture all but one of its cities--Japan dogpiles and takes a holy city before I can get to it, ugh.
Spain and Rome DoW on me soon after right as I am poised to invade the heart of Russia. You can guess what happened. I'm of course dying of laughter by this point at all these failed invasions-by-sea.
Now of course, I have the Great Wall, so all throughout this weird game in which nobody can attack me with land forces directly except my best buddy Toku (and he sure isn't going to attack me, not when we're the only Buddhists in a world full of heathens), I am spawning great generals since you get them double-speed if the combat takes place within your cultural borders. It's just getting ludicrous at this point; I have a dedicated Modern Armor factory that spits out a 17-xp Modern Armor almost every turn (military academy, heroic epic, theocracy, vassalage, barracks, 2 military instructors, West point, Pentagon, and a TON of waterwheels, workshops, farms, and mined hills), for instance. I also have 3 almost-invincible Modern Armor warlords by now, one of which is Combat VI, Drill IV, Morale, and Leadership enabled, with the others lagging a little bit in the Combat promotions department. I have another great general attached to Mech Inf, though that Mech Inf used to be an axeman back in my Egypt-pounding days.

So this is got me thinking that I should try to instigate some early wars with civs that hate my guts anyway, like crazy-religious Izzy. Obviously my situation was pretty sweet in that I was best buds with my buffer zone pal Japan and had a wall of mountain peaks to keep Spain at bay, but man, I can see the Great Wall being very useful on archipelago maps for builder types. With all of the generals I got, I could have just made them into military instructors and cranked out units that all START with 27 xp.
Also, Parthenon and Pacifism might work in conjunction with GG-making, but I'm not sure--anyone know? And an Imperialistic player would have another doubling. So I suppose in theory you could get a +100 GW bonus, +100 Imperialistic, +100 Pacifism, +50 Parthenon = +350% rate of GG making, or in other words, getting them 4.5 times more often than normal. I can imagine a city cranking out units that START with 37 xp.. now that's a scary thought.
(Edited to correct my unfortunate typo of "seige". Also because I wanted to be clear, in case it isn't obvious to any civ veteran, why you want only NAVAL invasions: if you declare war against someone who can invade you via land routes, you stand the risk of them pillaging stuff around your frontier cities. With naval invasion stacks, you can pound away on their one big stack with as much siege as you want followed by a mop-up crew. No pillaging, as the enemy soldiers just step step off the boat and immediately die.)