There are two basic reasons to connect your cities by roads: for Trade Route income, and for Unit movement
Connecting for Trade Routes
Warning! These calculations are now suspect because of new info found about how road maintenance works. I will update this with any changes needed once we have verified numbers for road maintenance costs.
Summary: only build trade routes to cities with 6 population or more. If you care for the math, then continue reading...
Now that Oak has solved the Trade Route formula for us, we know that normal trade income is very close to 1.25 * population of the connected city. (not the capital)
So, suppose you have your capital and one city, they are separated by 6 hexes. You'll be paying to maintain each of those roads, costing you 6g. So, at a population of 3, you would make 3.75g, be spending 6g, and be losing money overall. Your worker time is EASILY spent doing something else. Of course, if your cities are closer together, the calculations look better. Finally, I believe that Harbors cost 2g to maintain. Realizing that you only pay for your capital harbor once, sea routes effectively only cost 2g to maintain.
Population | Income | Net Income with...| Harbor/ |
| | 6 Roads | 4 Roads | 2 Roads |
3 | 3.75g | -2.25g | -.25g | 1.75g |
4 | 5.00g | -1.00g | 1.00g | 3.00g |
5 | 6.25g | 0.25g | 2.25g | 4.25g |
6 | 7.50g | 1.50g | 3.50g | 5.50g |
I would like to propose a rule of thumb. You can pick whichever population number seems best to you, but I believe my worker can do better than even a 4g improvement by working other tiles in the time it would have taken him to build that road. Thus:
Only build trade routes to cities with 6 population or more.
If you follow this rule of thumb, you will, at the least, never lose money on a trade route. Further, if you start working on the trade route when the city is size 6 already, there's a good chance it will have grown to 7 at least by the time you finish, and you can actually start making enough money for that worker time to be worth it.
Final note on trade routes: Arabia gets a flat +1 to trade route income. (AFTER the multiplier, so simply add 1 to every gold number on the chart.) So feel free connect Arabian cities at population 5 or more, if you like.
Connecting for Unit Movement
We don't have nearly as good of data on this, but from my Civilization experiences, roads are VERY worth building if you are producing units and sending them to war. You get your best units at the front for longer before they are obsolete, and you'll have more units at the front at any given time.
If you're not moving units for war, roads may also be worth building to quickly move defenders in the event of being attacked, by barbarians or another Civ. I think you can just ask yourself:
- Am I likely to be attacked in that city?
- Is the attack likely to beat the city?
- Do you HAVE units that could get there by the road to defend?
If any of those it no, the road is not worth it, cause it won't help.
And if you're not at war, and not defending, I doubt you want to build roads just to move units, because you don't sound like you're moving units around much at all!
The money gained from performing a diplomatic mission is 350 + (50 * <era number>)
where era number is just a serial number starting with 0 for the ancient era. In addition game pacing also affect this number: quick games modify this value by 0.67, epic by 1.5 and marathon by 3; or in other words:
Gold in Gold in Gold in Gold in
Era Quick Standard Epic Marathon
--------------------------------------------------
Ancient 234.5 350 525 1050
Classical 268 400 600 1200
Medieval 301.5 450 675 1350
Renaissance 335 500 750 1500
Industrial 368.5 550 825 1650
Modern 402 600 900 1800
Future 435.5 650 975 1950
I know of nothing else which can affect the gold amount.
Now, if I have a strong economy in the late game I typically gain another 100-200 gold per turn when in a golden age. That means that the only way in which a golden age is preferable to a diplomatic mission is when it lasts at least 3 to 6 turns. But usually by the time I get to the late game my golden age duration from great people is really low, and if you take into consideration that the influence gained is worth a few hundreds in itself, you'll see that a golden age looks less and less profitable in comparison. All this is especially true if you play in longer game speeds, which translate to higher gold.
Of course, golden age also nets production, so it's still a dilemma; but I do believe merchants are worth it from the financial aspect.
Best Answer
Friends can be useful in two situations:
(1) If your friend is powerful, it may deter one of your close neighbours from attacking you, both if they're your friend's friend, and if your friend is stronger than them. In a similar vein, if you made an enemy, and can DOF with someone who lives more closely to said enemy, the enemy is more likely to butcher your new-found friend first (and the demanded resources allow you to somewhat fight a proxy war).
(2) If you want others to declare war on you so that they may bleed themselves dry on that citadel you built on a choke point, or because you don't want to look like a warmonger to everyone, a DOF with their sworn enemy helps pushing them over the edge.