I have never won this game at Monarch level.
I've uploaded your save, and it says Emperor? And you did Ok, because:
I did a good job of grabbing just the right amount of contiguous territory, with lots of resources and good terrain.
You're in a spot of trouble now, I see that. I think you can easily become a much better player by just improving your awareness on a couple of things.
In this save, in my opinion, your basic issues are:
* Being behind in tech.
* Having too many defensive units (too few attacking ones).
I can't really know for sure how you ended up being behind in tech, but what you would normally do have a decent tech pace is:
* Research as horizontally as possible on the tech tree. Then use those techs to trade for the ones that the AI has researched.
* Check regularly what the AI has researched, so that you don't miss trade opportunities.
The Zulu's are not known for their research speed, but with a decent size empire like you've got here, and republic as your government, you should at least be able to keep up.
Then your military: too many defensive units. The thing is: often attacking units give you better defense, because they give you the possibility to attack incoming units on your own turn. Especially along your coastline defensive units are almost completely useless. What are you going to do if an enemy galley drops off a few Medieval Infantry? With Knights you can brush them away, Musketmen can only stand and watch.
You do need defensive units in your border towns that can be hit by enemy Cavalry in one turn, coming out of the fog of war. But your mix here isn't quite right, and makes you vulnerable.
I saw you let the governor manage your citizens. If you want to become a better player you should do it yourself. The governor does wasteful things. Proper micro management is a bit of a chore, but required on the higher levels.
Your city placement seems fine to me, but your cities are not used in the most effective way. You're not specializing properly. What I saw is that you had cities without a food bonus with granaries, and cities with a food bonus that had no granaries.
Cities that have a food bonus are better at producing growth; they're better at producing workers and settlers, and a granary improves that quality even more. Cities that don't have a food bonus you're likely to use for unit production. Then you would build a barracks here, and no granary.
I saw some towns (Intombe, Umtata, Isandhlawa) that were producing units, but had no barracks. Barracks give your units an extra hitpoint, making them stronger, and the chance to create leaders gets increased significantly. Well, it's mainly attacking units that will generate leaders. This is another point for attacking units. I also saw a town with a barracks (Mpondo) that was making a cannon. Artillery doesn't get an extra hitpoint, so they don't profit from being built in a town with a barracks. Artillery you would normally build in towns without barracks.
Ulundi and Zimbabwe had a colosseum. Colosseums are very expensive for what they give. If you're not going for a cultural win then the shields are better invested in something else. I believe both towns had a barracks as well. If the barracks were there earlier, you should have mainly built units there. Use the lux slider to keep your towns from rioting, or try to capture more luxuries.
So there's room for improvement, but I don't think it would take you too much to move up from here.
Edit: I forgot something; you said you didn't expand any further because of corruption. That should
never keep you from expanding. Yeah, you're corruption percentage goes up as you grow bigger, but those extra towns are still helping you!