This is a story I didn't know - a small group of Islamic adventurers took over a small fortress in Southern France and set themselves up as a local force.
It's detailed in this post: Fraxinetum.
While Matt does outline some ways of playing it - PCs + Allies / Ally Group, I think it has a lot of possible ways to play it.
Here are two I like:
Instead of an Ally Group, make each player an Ally Group. Allow each player a main character, a few secondary (lower point) types, and a lot of minor PCs to support them. This is essentially how you'd run a Pirate crew in Yaquinto's excellent Pirates & Plunder RPG, a game that had an enormous influence on me.
I've run a game like this in the past, and it's fine - you rotate through your main characters and backups as necessary, and fill out groups with PC-made extras who fulfill the roles of NPCs without being generic Allies.
Black Company Style
The Black Company books are really books about individuals and small groups, for the most part. The encounters between the main force of the company and obstacles aren't sidelines, exactly, but it's rare you know the exact size and makeup of the group. It's more like a formless mass of approximate size that provides a basis for encounters and a pool of PCs and NPCs.
With this kind of game, you'd essentially put aside mass combat, and emphasize the small group encounters and individual actions of the PCs. You'd have a pool of associated warriors to provide replacements for killed or crippled PCs, for reinforcements, and to explain why the PCs don't have to do boring stuff like garrison or run messages or other annoying logistical bits. Or do the actual logistics.
With either of those or Matt's suggestions, I think this is a good historical example of something that seems pretty gamey - bold warriors seizing borderland, attracting followers after they do so, and then ruling the area until civilization essentially co-opts them or forces them out. It's the classic "end game" of D&D, except it's how it should be - the start, not the finish, of adventure.