On the whole i like the map, specially because it's about a historical event that has taken place not too far from where I live.
Here some initial thoughts
I would suggest you to enlarge the battle area, to make clear the connections and to be able to add some nice graphics later, when in the graphics stage.
If I look at your map I read that RM lines can attack the Cartaginian Cavalry Units on both sides, well it's probably accurate from an historical point of view, but it's not so clear looking at the map itself. Infact if I look at the map I think that from RM4 I have to move to north and go through RM3,RM2 to reach RM1 and then RH1, but th elegend let me think that i can attack directly RH1 from RM4. If use more space for the battleground, instead of use it only for legend, backstory, it would be easier to gather the map while playing without having to read the legend all time.
Then, I understand the gameplay reason behind bombardments, but why cavalry can bombard roman lines? I know elephants were used to throw arrows in battles, but their strong point was to use the mole of this animals to cause terror in the enemy lines and break their lines. If i have an elephant i would go directly against my enemy, specially if he is just an infantry unit. On the opposite i see roman cavalry was used in the right way and I like the one way option.
From a pure gameplay point of view, so without considering the backstory events, the major flaw i see is the fact that in the way you set the starting positions will surely push players to stack armies and try to kill opponents without go through the decay area. So probably in a 8 player game it will end with 2 players, one on the ambush side and the other on the front side that stack and wait to kill each other. You should consider all possible settings when you try to develope a map, so if you think about a 8players escalating game, or worst to a assassin game....imagine if the player that start in CR4 has to kill the one in CR3....i bet it won't be a so fun game, maybe quick, but certainly not funny....at least for those who will lose in few turns.
My suggestion is to try to "force" players to fight instead of stack, maybe you can force the hand on history and add cartaginian and roman camps and make this map a just 1vs1 map, give to player the possibility to create a different historical ending for this famous battle. Make it a sort of Carthago vs Romae map, if you get what I mean.
Instead if you want to keep it as a multiplayer map (from 2 to 8 players), then you certainly have to change a bit the positions and , like I said previously, force the players to go throught the middle, maybe give them an incentive...because right now stay in the middle gives to the player only disadvantages (decay, killer neutrals)
These are my very initial thoughts looking at the map, I'll try to give you more inputs soon...but on the whole it looks a nice project that can come out nicely if developed in the proper way!
Keep it on!