How do you defeat runaway civs on other continents?

For (fairly) early land warfare I find you can't beat the combination of cannon and riflemen. Frigates, especially with a range upgrade, will destroy coastal cities, and don't forget that although there is a penalty (without the promotion) you can attack with land units from the sea.

PS. I love all this advice from people claiming to play on Emperor that won't even work on Prince. Even on prince the AI will build horsemen or spearmen that will easily counter your far more expensive to produce swordsman. I am not saying I have all the answers, I don't claim to play at Emperor level either though

Cheesy, but you can just let them run away, even help them, prepare for a diplomatic win and let them build the UN.

Ah yes, cannonae and riflemen, my favorite staples of late Renaissance warfare.

I once had the drastic problem of having to take out a problematic neighboring runaway who was blocked by... difficult terrain.



He had something like two Frigates and a caravel on the prowl, while I only had a lone Caravel to keep watch.



It was amazing.

Most likely you're teching wrong. It's possible to enter info era at t260 without any RAs and almost any start if you choose which techs to beeline and how. Try to reach these techs by these turns:

1. Education by t110
2. Scientific theory by t175
3. Plastics by t220

Normally you'd need to shave about 10 turns off these times to compete on deity, but we're talking emperor/immortal here, so you can be a bit lax. If you want to go domination VC, there are some guidelines you should follow.

Here are a few more science-specific tips:

1. You should have 3-6 core cities set up by t100. Also, NC is recommended t90 or earlier.
2. In core cities, population should be 10 by education, 15 by sci theory, and 20 by plastics.
3. If it takes more than 10 turns to build a science building and you have a decent/strong economy, buy those buildings. Otherwise save cash for unit upgrades - they're more expensive than you think.
4. Try to build PT and LToP; get great scientists for both. Scientists attained earlier than this should be planted; these scientists should be saved for bulbing.
5. Enter modern era by building Oxford and getting replaceable parts as free tech.
6. Bulb one GS for radio, another for plastics.
8. Get artillery; shouldn't be too long with labs and factories.
9. Enjoy as your technologically superior units crush rifles and muskets of other civs. Build frigates and upgrade to battleships; 4 should be enough.
7. Later, use rationalism finisher for rocketry and satellites.
8. Should any other runaways remain, use 2 GS from Hubble to get mech inf/nuke subs, tech/build nukes, and mobile SAM spam their air force to death.
9. tl;dr: get science techs and build/buy science buildings ASAP. I know cannons/rifles might seem enticing, but waiting ~20-30 turns for units that are more than twice as strong is more than worth it. Also, grow grow grow!

What units you should build, and when:
1. Early in the game: 4-6 ranged units and upgrade as necessary.
2. A pike, as they are stronger and more resistant to ranged attacks than swords.
3. Mid-game (medieval-renaissance): 4-6 melee units and 4-6 siege, to upgrade to artillery/infantry later. Caravel for exploration.
4. Later: naval units, bombers, mobile SAM spam, etc.

A word about defense:
In G+K, everyone is your enemy, but that doesn't mean you have to spam units to stay afloat. Learn to bribe enemies to attack each other, pick on weak civs to get work-together diplo bonuses, and to attack/defend with 4-6 ranged/1 melee unit. See Madjinn's videos for advice on early-game DOWs.

That's all the advice I can give. Hope this helps!

If they got powerful from conquering other civs and wiped them out completely, you can get back at them by invading by starting with just the cities they conquered. Choose to liberate those cities instead of keeping them (unless you really need them) and that civ comes back from the dead in love with you. Now you've got a life long partner for doing Research Agreements (though you may have to forward them the money).