*Please* don't go down the non-combat route. I know it seems appealing as it's sort of not been done before and its a challenge, but I think it'll lose a ton of appeal. Mainly though, I feel it would be a bit bizarre/less realistic because you couldn't attack anyone in a time where life was pretty violent. I mean, as everyone carried around swords or weapons, and there are guards/militia that are obviously armed - it'd be a bit odd. If you tried to force it by making the player a pacifist character it'd lock out a lot of roleplaying options and stop the open play style which URR is heading for.
I think that people tend to get a somewhat distorted idea of the past (since we tend to have movies, games, and books about it that fixate on the "exciting" parts, which usually means combat.) Most people in the era when Ultima Ratio Regum was set would not carry weapons. In fact, depending on exactly when and where it was set, most people might not even have been
allowed to carry weapons. They might have had armed guards -- but then again, they might not have. Smaller settlements likely wouldn't have actually had any formal police or guards at all, they're a modern innovation. If you count police, you'd probably have a bigger "armed militia", per capita, in most cities in the modern US than in most cities in the ancient world.
The vast majority of people throughout history have gone through life without killing anyone. (There are a few exceptions -- Sparta systematized the murder of its underclass of slaves in order to keep them in line, say -- but even then it holds true, since most of the people there would have been helot slaves; and such societies continue to attract interest because they are
unusual, that is to say, not normal, even in their own time period.) In the Middle Ages, the homicide rate was a bit higher than it was today, but emphasis on a bit -- in 14th century London, for example, the homicide rate was roughly 36 to 52 per 100,000 people per year, not that far off from what it was in the nastier parts of New York's recent history.
Anyway, I'm mostly indifferent to whether violence is in the game or not as long as it's not the main focus; the main question is whether it can make doing other things (diplomacy, exploration, and so on) fun, giving players ways to interact with the world in a way that makes the things they learn about it meaningful.