From the screenshot I see you have three of your 10 cities building great wonders. Do you really need to spend all those shields for those wonders?
(downloading and analyzing the game...)
Sun Tzu, 600 shields, gives you a free barracks on every city on the continent. Barracks cost 40s. You have 8 cities on the continent. 8x40 = 320 shields to build native barracks in each city. Granted, each barracks would cost 1gpt, but really you don't need a barracks in each city. London, Nottingham, Canterbury and Hastings are your high-production cities. (Holy crap, an aqueduct would have been a much more worthwhile build than Sun Tzu in Hastings! You're stuck at size 6 in that wonderful city with hills and grass to work.) Anyway, I would have put barracks in those cities and build all troops from there and probably would have owned more of the continent by now.
You have a longbow standing on Saltpeter just outside your territory. You know about colonies, right? This is one of those situations where you should move a worker there, establish a colony and research Military Tradition ASAP. You're only two techs away but are researching Navigation instead (Why Navigation? You're going for Magellan's, aren't you?
You can already circumnavigate the globe without leaving sea squares. Magellan's (also Navigation) does nothing for you on this map unless you're going for culture or wonder victory.) . If you fear having spears for defense and not pikemen you can build musketmen now (if you colonize the saltpeter), but I'd lean towards rushing to Military Tradition and Cavalry and go all-offense and roll Korea who...holy crap they're just two techs into the middle age and can't build Knights yet!
You have two barracks built, but most of your troops are regulars. In fact I see only 6 veteran land troops. That extra HP makes a worthwhile difference. I try to identify high-production cities early in the game and usually have them build barracks early and supply the empire with veteran troops.
You have 7 Caravels, all fortified. What's up with that?
You're actually in a very good position. Seriously, I would abandon the Navigation research, beeline to Military Tradition, colonize the Saltpeter and roll the world with Cavalry carried to the other continent with your 7 Caravels. I'm not sure I'd even bother to leave the cities standing or resettle the conquered lands as it would just slow me down. Nobody else can build knights yet. Also , the other continent doesn't have iron hooked up yet, so they're still running around with archers, spears
and horsemen...they don't have horses connected, either!
Edit: Oh I see you say you got clobbered fighting Korea with muskets, so I guess you lost a city or two? The military adviser says you're average compared to Korea. You probably absorbed the initial rush and failed to continue pounding. Often after the initial rush of units they are gassed and can't keep up with your production, epecially on Warlord. With a strategic resource disadvantage you just need to pick your battles more carefully, especially since Korea has/had ancient cavs. Or you could have traded with them and built a bunch of swords or knights (depending on when) during the 20-turn period and then used them to take the iron permanently. Or have a bunch of units ready to upgrade, trade for the iron and mass-upgrade. Bingo, instant strong army. But you're less than 10 turns from Cavalry, so resource disadvantage is no more.
Edit 2: Muskets are kind of expensive for what they're worth at that point in the game. And defensive units give the enemy the choice of where the fight happens. Offensive units let you choose where the battle is. If I were warring in the Gunpowder age in your situation I think I would have schemed to get a bunch of knights and used them for offense and defense.