You have a couple of options. First, there's the peaceful option. Keep expanding. You have tons of room left -- the more of it you can take, the more powerful you'll look to the AI and the less likely they'll be to attack. The Great Wall is available; if you use your scientific leader for that and build spears, catapults and the odd archer for mopping up any leftovers, you'll be almost invulnerable to attack, even once knights are running around. Just wait it out and see if you can trade for iron or horses from someone later on. If you keep hold of your tech lead, that shouldn't be a problem. Your base commerce is pretty good
Second (and my preference), there's the military option. I see iron right by Theveste, nice and close by. Build about ten archers -- your cities are good sized now so that shouldn't take too long -- and send them over with a couple of spears for defense. As long as Theveste doesn't get above size six, you should be able to take it without too much loss, even with 3-defense units defending. (They won't have more than 2 defenders for a while, and if you attack there first, they won't be able to reinforce effectively in time.) If you have the units to spare, go on and take the dyes town; if not, just hold them off until they'll talk, and make peace. There's your iron. (If you go this route, I'd probably use the leader for the temple of artemis of the options currently available -- there's nothing like free culture expansions to push your borders out nice and fast).
A couple of comments --
This is really rare for me, but I'd recommend you halt research on Republic and trade for Monarchy instead. You have only one native lux, with many turns to go until the ivory is hooked up, and only the dyes within range to take militarily. Republic won't be fun with only one lux on line, and won't be much better even with two. You can also get monarchy now, instead of in 20 plus turns, not a trivial difference.
You do, however, need a ton more military police. There's no way you should be running 40% lux right now, and no way your largest cities should have less than two units apiece. Take advantage of the fact that you don't have iron yet and build a bunch of warriors, reg or vet, doesn't really matter. Once you have monarchy, your largest cities can get three units each for military police, which should really help with your happiness. You're still way under your unit limit, so no worries there.
In general, you have way too many defensive units for this point in the game. If instead of those 13 spearmen, six warriors and one archer you had built 20 warriors, six archers and one spearman, you'd have not only enough MPs to go around, but also be just about ready to go attack Carthage. As long as your empire is relatively large, the AI will rarely attack while there's still room to expand, so there's no real need for defense. A good offense is the best defense, anyway.
The libraries you're building are more-or-less useless right now; you're better off with a temple (or the temple of artemis) if you want the culture expansionin St.Regis, since it's cheaper. Alternately, you could just build workers and military police out of that city. Grand River should build military and workers, with all that food.
Finally, I noticed you have a worker or two chopping wetland while there's unimproved grassland right next door -- big no-no. Improve the easy tiles first. Wait to do the big projects until you can afford to devote several workers at once to the same job -- typically somewhere in the middle ages.
Good luck,
Renata
