@Lion.Kanzen @niektb if you are going to proceed, please consider continuing work on the same map, rather than starting a new one. This way, even if you won't finish the map, there still will be a progress in the same direction.
I've updated the latest version published here: [1] using a18_to_a19.py and replaced missing celt_* entities on @Yodaspirine's part with the correspondingly-looking gaul_ and brit_ ones. Here are the resulting map files: community_map_a20.zip
The only issue on @Yodaspirine's part I see is the rotary mill which has a different model and obstruction size now.
Roman camp on @idanwin's part looks the same, gaul buildings are changed (houses can be replaced with brit ones to look like the original ones; I left them as is, since they were initially added as gaul ones, rather than generic celt). Also cage actors are missing in both Alpha 12 and Alpha 20, I guess they are the models from this topic: [2]
Images: