Got a few games of hotseat played with Deity settings, which basically means you get no bonus versus barbarians without Honor policy when there are no computers (not sure if City States get the bonus but they're never aggressive anyway). The game is devoid of strategy until the Renaissance era. The only way you can ever take down a city early on is if you grossly outtech/outnumber/outXP the enemy, or you have the iron units while he doesn't have either iron or horse. Until you get the Musketman line of units, you simply don't have a unit that can remotely fight against a iron unit. Also siege weapons are usually too weak to punch through a city defense. The siege units are presumably scaled with wall structures in mind, so by the time you hit cannons you'll be able to find some cities that don't have a castle and actually do significant damage to them. For the game to have any meaningful depth you really should start at the Renaissance era as at that point there are actually decisions to make on whether to expand or not, what to build and what paths to pursue. In all my games, nothing ever happens in the period before Renaiisance era. You're pretty much completely incapable of taking someone's capital city unless you're way ahead of them in tech. In one of my game, the Iroquis are sandwiched between two of the world's top powers (Americans and Ottomans) and they have no iron and horse, but actually managed to hold their capital with crossbowman + mohawk warriors until cannons arrived. In the Renaissance era you also got a lot of interesting tech coming up that can break a stalemate which makes the research actually interesting.
The combat system in this game is pretty messed up with minimum damage set at around 3 unless you're grossly outclassed. This means a range unit, even when it is significantly weaker than a melee (e.g. your range attack is 30, their range defense is 50) you can still do 3 damage to him. This means having 2 range units with double attack can take out a unit that can beat you 10-1 or even 20-1 if it actually got close, so really the unit is way better than you but because of the minimum damage you can take that unit out. I put units on 20 HP which helps a lot with this issue, since the melee unit will kill your unit 20-1 whenever it gets close so the extra HP don't help you when your outclassed badly. That said range units are actually surprisingly durable and 20-1 outcomes are actually pretty rare.
The units still take way too long to build. It seems like unit cost are designed with stuff like Factories in mind because there's no way you can even produce simple units in a reasonable time. Most of the units cost as much as a building, and I know you can spend the whole game just buliding buildings if you've a decent research rate. I turn them to 1/2 cost in my game so you actually see units amassing, and I put the upkeep modifier to 4 (from which lowers the cost somewhat (it's some kind of curve so it's not exactly half), because otherwise you'd never be able to afford your army.
Some interesting things that pop up when all the players are human...
1. Money is actually hard to get. You don't have an AI to rob. The only kind of deal I do with myself is Research Agreement since both parties are locked into that. I never trade anything else because just about any other trade can be abused in some way.
2. Defense Pacts cannot be seen on the diplomatic screen and basically leads to a one person against the world if you dare to declare war. This is because signing DPs are almost always mutually beneficial unless you're planning to declare war on the guy you just signed a DP with. Even if you know that guy is about to get a Declaration of War by the most powerful player, there is no reason to believe the most powerful guy will leave you alone just because you ignored him, so you might as well do it. Unlike computers, a DoW is more like the natural state between human beings and people won't really think worse of you for being perpetually at war compared to peace. If anything having a Declaration of War makes it slightly harder for the guy to launch a sneak attack since the game will warn you when enemy units are approaching.
3. Research Agreements are the deterrent to war, though if you throw RAs with DPs you quickly end up with a world where everyone is at peace with everyone else, and when someone declares war it he ends up declaring war on the world so nobody would declare war. Unlike against AI, nobody's going to purposely sign a RA to make you lose money since money is hard to come by without AI to rob. Money is just too valuable to be wasted like that, not to mention the tech boost is huge.
4. City-states are much harder to keep since money is limited, and if you spend a ton of money on a valuable CS expect that guy to get conquered. After you declare war on City-States a few time you get the Warmonger penalty and at that point you might as well just declare war on all of them. The AI sucks anyway so it's almost like free XP for your units, and even at permanent war they're not very aggressive at attacking and will inexplicably fail to take even the weakest of cities. I had a guy with a city strength of 25 surrounded by a city state with Rifleman (25) and cannons (24?) and the city should've been bowled over immediatey but the AI kept on walk his cannon to point blank range to die and the Rifleman decided to go swimming in the lake nearby. Also, CS is a huge disadvantage if you've any allied, because if you want to protect one you got to declare war, but then due to #2 as soon as you declare war on a player you're now at war with the world. Even if you limit the DPs, having a city-state ally actually makes you an attractive target because someone can declare war on you, which makes the CS declare war on them, and now they can conquer the CS without the Warmonger penalty. Again unlike against AI you don't carry any extra penalty for being at war with someone but the city-states do. Puppeting cities is pretty much the best way to expand early on as you don't have to worry about culture or National Wonder costs, not to mention most CSs are usually sitting on some type of strategic/luxury resource. I think in a human player game it'd probably be best to use no CS because having any allied to you is actually an open invitation for everyone to declare war on you so they can attack the CS without any penalty.
The combat system in this game is pretty messed up with minimum damage set at around 3 unless you're grossly outclassed. This means a range unit, even when it is significantly weaker than a melee (e.g. your range attack is 30, their range defense is 50) you can still do 3 damage to him. This means having 2 range units with double attack can take out a unit that can beat you 10-1 or even 20-1 if it actually got close, so really the unit is way better than you but because of the minimum damage you can take that unit out. I put units on 20 HP which helps a lot with this issue, since the melee unit will kill your unit 20-1 whenever it gets close so the extra HP don't help you when your outclassed badly. That said range units are actually surprisingly durable and 20-1 outcomes are actually pretty rare.
The units still take way too long to build. It seems like unit cost are designed with stuff like Factories in mind because there's no way you can even produce simple units in a reasonable time. Most of the units cost as much as a building, and I know you can spend the whole game just buliding buildings if you've a decent research rate. I turn them to 1/2 cost in my game so you actually see units amassing, and I put the upkeep modifier to 4 (from which lowers the cost somewhat (it's some kind of curve so it's not exactly half), because otherwise you'd never be able to afford your army.
Some interesting things that pop up when all the players are human...
1. Money is actually hard to get. You don't have an AI to rob. The only kind of deal I do with myself is Research Agreement since both parties are locked into that. I never trade anything else because just about any other trade can be abused in some way.
2. Defense Pacts cannot be seen on the diplomatic screen and basically leads to a one person against the world if you dare to declare war. This is because signing DPs are almost always mutually beneficial unless you're planning to declare war on the guy you just signed a DP with. Even if you know that guy is about to get a Declaration of War by the most powerful player, there is no reason to believe the most powerful guy will leave you alone just because you ignored him, so you might as well do it. Unlike computers, a DoW is more like the natural state between human beings and people won't really think worse of you for being perpetually at war compared to peace. If anything having a Declaration of War makes it slightly harder for the guy to launch a sneak attack since the game will warn you when enemy units are approaching.
3. Research Agreements are the deterrent to war, though if you throw RAs with DPs you quickly end up with a world where everyone is at peace with everyone else, and when someone declares war it he ends up declaring war on the world so nobody would declare war. Unlike against AI, nobody's going to purposely sign a RA to make you lose money since money is hard to come by without AI to rob. Money is just too valuable to be wasted like that, not to mention the tech boost is huge.
4. City-states are much harder to keep since money is limited, and if you spend a ton of money on a valuable CS expect that guy to get conquered. After you declare war on City-States a few time you get the Warmonger penalty and at that point you might as well just declare war on all of them. The AI sucks anyway so it's almost like free XP for your units, and even at permanent war they're not very aggressive at attacking and will inexplicably fail to take even the weakest of cities. I had a guy with a city strength of 25 surrounded by a city state with Rifleman (25) and cannons (24?) and the city should've been bowled over immediatey but the AI kept on walk his cannon to point blank range to die and the Rifleman decided to go swimming in the lake nearby. Also, CS is a huge disadvantage if you've any allied, because if you want to protect one you got to declare war, but then due to #2 as soon as you declare war on a player you're now at war with the world. Even if you limit the DPs, having a city-state ally actually makes you an attractive target because someone can declare war on you, which makes the CS declare war on them, and now they can conquer the CS without the Warmonger penalty. Again unlike against AI you don't carry any extra penalty for being at war with someone but the city-states do. Puppeting cities is pretty much the best way to expand early on as you don't have to worry about culture or National Wonder costs, not to mention most CSs are usually sitting on some type of strategic/luxury resource. I think in a human player game it'd probably be best to use no CS because having any allied to you is actually an open invitation for everyone to declare war on you so they can attack the CS without any penalty.