An interesting, if very weird, film. I like it more now after a second viewing. I've never seen Damon better-- up there with Johnny Yuma as his 2 best.
The setting of the fort (according to the map in the film) would be present-day Cotulla, Texas, a small town founded by Polish immigrants. And I think that the conflict depicted was a genuine, historical one, something not always seen in a SW. After Texas' independence many of the Spanish/Mexican landowners (holding old land grants) were dispossessed by the Texans (gringos). A real injustice, especially considering that so many of the Texas patriots were ethnically Mexican-- Juan Seguin, Jose Navarro. That occurred well before the Civil War.
Lizzani and Pasolini turned it into a "communist" struggle, but what the heck. I've never before seen a film like this, so it doesn't bother me.