There are pros and cons to raging barbs.
Doesn't it make the game really hard?
I wouldn't say that, it just means you aren't allowed to run minimal military at any time. I find it usually keeps people's power charts higher than say if they were turned off.
I mean, won't the AI increase their military?
I usually mix this with Agg AI as it will help them know they need to spam units more than usual to capitalize on this.
Won't I have to belay workers for more warriors/axes/chariots?
I would say it adds an emphasis on the use of slavery. But if you can get a few good places around your territory to fight them off, you don't need too much military. (This is common enough to not be called a rare circumstance.) Use hills, rivers, and peaks for defensive positioning as normal.
Or does it make it easier (quick Great Wall grab)? Or is it more fun?
I would say yes to both. But honestly, depending on map size, the GW can obsolete pretty early. (I mean once the land is settled.) I have found a couple games where I was getting plowed by barbs and the GW saved my butt and put me in a much stronger position. But those times are rare and usually a result of something else I did in combination with pulling off the GW in time. If you are planning to rush and can somehow work in the GW without slowing your rush, it rocks. What is better than the no barbs aspect of the GW is the GG emergence in your borders. That lasts for a long time and if you play off of it right can be very rewarding. Imperialistic + GW = fun.
I have still yet to play the Terra map, but it seems that it would play different than raging barbs on say tectonics. But it is a more controlled environment which may be why you are using it.
Do an axe rush. If you pop HBR from a hut, do a HA rush. Or mix em. If all you have is horses, just use chariots as a strong border patrol army. Prepare 2-3 Chariots to guard each future city location. (As you build the settler) Then bring an archer from somewhere to the new city. Rinse and repeat as you expand. (Up to you if you want to make more chariots or use the ones you already have and backfill with better city defenders.) I would use C1, shock, C2 for the chariots to give them some effort when swords show up. But if you dont have copper, prioritize iron fairly heavy. Same rules apply to HA's and axes for border patrol even while your rush is going on.
The cool thing about raging barbs is sometimes they spawn a city right where you wanted one - or close enough. But the AI is out hunting these cities too if you set Agg AI. 1 or 2 will also probably die in the beginning.
EDIT: What difficulty are you gonna play on?
EDIT 2: I don't like doing chariot rushes unless I am Egypt or Persia. So some of this is also subject to what civ you play as.