I normally play noble but with these interesting conditions, I thought I'd give it a try. After some failed tries I managed to time things right (and probably had a little luck too).
As others said, archery first is top priority. I worked the river plains while building a workboat, once it was done I switched to work the crab. By that time I had walked my warrior around the city and now returned him to survive against the first barbs. I think I got the first archer out on turn 16 or so, my warrior defended the city against 1 barb warrior. After the archer was out, I moved the warrior to the east and then south east, and parked him on the coast to fogbust the eastern area. One of the next archers I moved to the north west. So, after that the main attack line for barbs was from the south.
Meanwhile I built a worker and cottaged the two grass tiles north of London. This turned out a good thing, they were never attacked and pillaged.
However I failed to build one (or several) STRONG team of several archers and fortify them on the hills to the south. Only tried to build a single or two archers several times and send them, only to lose them quickly. Because of that, I could never build the pasture on the horses.
Eventually I built the first city to the east, and another to the west (on the sheep hill to capture the copper after border pop). I had also been researching Monarchy for a long time, somewhat useless because I had hooked up the fur resources. My science was down to 20% or so, building another city would've been possible I guess but then? That was the situation around 200AD. I think I need to improve my gaming more before I can really handle deity maintenance
Lessons learned for another try: build multiple teams of archers early and send them out as soon as possible, so the horse pasture can be build safely.