Very topical! I've just finished a novel about the same period 55 BC entitled Iron Blood & Sacrifice (The Sons of Beli Mawr).
The remaining history is fractured and contradictory at best, making a reasonably accurate stab at an historic event problematical. The map you show was part of the inspiration and my early research. The storyline of my novel is based around the known happenings in this time-frame and I hope I have captured somewhat, the wild nature of the period. I have used too where possible, the main character’s names, who were real enough, so are the strongholds I’ve chosen for the tribes and the locations for the events that unfold. My stubborn adherence to the historic timeline caused many sleepless nights but I got there in the end. The liberating thing about the paucity of information and Non-Roman resource, is that it allows for a great deal of artistic license and although it became a bit of an obsession, I really enjoyed the process. Looking for test readers currently, so if you're interested in a historic romp with a very 'Welsh' twist, please contact me and I would be delighted to share the MSS.
Regards,
Eifion