Regions that cannot be taken with the Celtic army list

[benzidrine]

I love the game so far and have gotten quite far in a game as the Averni. In the current game I have really no challengers and pretty certain to victory at this point I would think.

One thing that is odd to me though is that I have an enemy reduced to a single mountainous region and it appears that I cannot take this region with any army list available to me due to the limited frontage and palace guard. I have attempted multiple infantry and horse compositions and even experimented with sending in waves of armies in a single turn. Also multiple turns of harassment with skirmishers. I never kill them or effect them at all.

While it doesn't really effect the game's outcome it is an annoyance because since they can build up an army they can essentially go rampaging through at will without me being able to stop them. I have fortified all surrounding regions but they tend to just walk past them until they get to the interior.

It feels like a game design problem at this point.