If you want to launch a surprise attack on the Gauls, you might want to consider getting in on Hegemony Rome: The Rise of Caesar’s early access. It’s Longbow Games’ latest in the Hegemony series (the previous title focused on Ancient Greece,) offering similar real-time-with-pausing mechanics to that previous game.
In Hegemony Rome, the overall world map, closer tactical map and battlefield RTS view are all part of one fluid system. If you zoom right out, you’ll get a grand overview, while zooming in might allow you to follow raiding parties and troop maneuvers. In Rise of Caesar, that campaign map stretches from “the Mediterranean coast to the shores of Britannia.” Supply lines and logistics are crucial to success.
The early access version of the game costs $25.00 USD (that price will go up as it nears full release,) and includes the first two (of four) chapters of the Caesar campaign. Hegemony Rome will later add a sandbox mode, in which you’ll be able to command other factions besides the Romans.
As usual with early access titles, be aware what you’re buying. This is an unfinished version of the game with accompanying bugs and balance issues. But, if you desire, you’ll be able to offer the developers helpful suggestions and bug reports to help them fix things.
Here’s a video from Longbow explaining why they opted for early access.