I had a problem: I needed a new combat system.
The set piece battle sub-system I used in Agricola, Master of Britain and Charlemagne, Master of Europe involved deploying your troops along one side of a staggered square grid opposite randomly drawn enemies. In a series of alternating attack and defense rounds, each of your units would roll against the enemy. Once you had made your initial deployments, your options were limited to a choice of adjacent targets during the attack rounds, and so the battles were basically resolved on a sort of auto-pilot roll-off.
Reaction to the system in Agricola was mixed, and so when I did Charlemagne, I added some more nuance to it, dividing the opposing armies each into two wings, with the routing of a single wing winning the day. Special scara units had certain advantages in combat, and could be redeployed from one wing to the other. You still had very minimal control over how your troops performed, as the game wasn't really interested in your tactical prowess. Instead, the game was testing your ability to build, maintain, and manage an army and the costs associated with it.
Reactions to the Charlemagne iteration were also mixed however, and so in designing Aurelian, Restorer of the World - the shorter, snappier cousin to those two earlier games - I decided to junk it entirely. Which leads me to that problem: I needed a new combat system. It needed to be much quicker and simpler - a single die roll instead of two or three dozen - while still being a test of the player's ability to build up and manage their army. Good army management would result in fairly easy, even more-or-less inevitable, victories, and bad management would result in things being more likely to go agley.
As usual, the subject itself suggested a solution. Aurelian, like many emperors of the period, had to deal with incursions by Germanic tribes. So instead of concentrating on building up your combat troops, you would instead be building up your defenses along the Danube. When a "barbarian" unit is drawn, a die roll will determine which of the eight Danube spaces it will attack. At that point, the unit posted to that space must roll a die, adding its own strength and the strength of any Walls (+1 or +3) you have built within the Region. Roll higher than the enemy, and you push them back across the Danube. Roll lower, and your unit is eliminated.
The lowest modifier you could get would be a +1 (one-strength Legion with no Walls built), and the highest a +6 (three-strength Legion with a completed +3 Walls marker). The barbarian strengths range from four to seven. As a result, a +6 roll on the d8 would only fail against a 7-strength barbarian, and then only if they rolled a one. By comparison, a +1 roll would have a much harder time. Hey, let's look at a chart!
Players start with 12 legionary strength divided equally among six Danube spaces. One of the actions available to the player, a Redeploy action, allows them to redistribute this strength, to a maximum of three and minimum of one per space. Often it is common for a player to open the game with this action, leaving themselves weaker in some areas and stronger in others, as a sort of calculated risk. They're also likely to move some strength into their mobile army that accompanies the emperor on his campaigns (hey, that empire isn't going to reconquer itself).
This army begins the game with a strength of 2, and maxes out at 5. When Aurelian takes a Campaign action - sallying against the barbarians on the other side of the Danube - or a Battle action - against units in rebellion - the player will add this army strength as well as Aurelian's leadership modifier (+1 or +2) to their d8 roll. The player will always "win" the battle - that's never in question. But the battle might result in them losing army strength, and if this is reduced to zero, they lose the game. As with the defense rolls on the Danube, the game is weighted toward the player's success.
So, the chances of losing your entire army are pretty slender. There's a greater chance that you will lose at least one Army Strength from a single Campaign or Battle, but it's not much greater:
You'll note a predominance of "12.5%" in all these Campaign and Battle charts, and there's a simple reason for that: a natural roll of one always counts as a one, regardless of what other modifiers will apply. If that wasn't the case, the chances of losing at least one Army Strength from a battle or campaign would be about the same for smaller armies, but greatly reduced for larger ones when fighting against smaller enemy forces.
Without the natural one rule, certain outcomes would become deterministic, and the player would be able to "game the system" to ensure they never have to deal with attrition. And while the natural one rule allows for this fuzziness, the system is still heavily weighted in the player's favor, and the player in many cases makes their own luck. A large army picking fights with smaller enemy forces is more likely to be safe; pitting a small army against a larger force is a recipe for disaster.
Of course increasing the strength of your mobile Army means decreasing the strength you have along the Danube, and vice-versa. Judicious use of Campaign actions will help with that, as half of the defeated barbarians are recruited into the Legions to defend the Danube. It also gives you more money, which you can use to build those walls you need, among other things. Of course, you'd want those enemy forces to be as large as possible to maximize your potential gains - which also increases your chances of a bloodied nose or worse.
This makes the game one of calculated risk rather than a random dice fest. One of the most important things to me in my solitaire designs is to give the player a large degree of agency. While there isn't a way to directly mitigate a lousy roll, it is the player's ability to manage their resources that determine their odds of success in the first place.