The Shot & Shell Battle Series originated because I had designed three previous games that had somewhat similar rulesets but at different scales, with different combat procedures and different ZOC rules and different activation mechanisms (an I-go-U-go for this one, chit pull for that one, activation roll for this guy over here). I couldn't really call those three previous games a "series", because for me as a gamer the appeal of a series is that you only need to learn the rules once, then you're off to the races. Having to learn each game separately, and if you happen to own more than one, having to keep them all straight (in this one cavalry works like this, while in this one they work like that): that stuff's for the birds.
And as a designer, the appeal of a series is that I only need to do the heavy-lifting the first time. Each new game might have its own little rulebook that adds or subtracts from the whole, but that takes exponentially less effort than reinventing the wheel (and writing a new rulebook) every time I want to do a game in that vein. Call it laziness as motivator.
Now, I actually wouldn't call myself a lazy person necessarily; in fact, I rather enjoy work - not just the work I'm doing, though it is very much the bee's knees, and maybe even the bee's elbows, but work in general, work as a concept, work as an ethos. When I'm not working, I get restless and agitated. Solution? Work, work, work. Perhaps because of that however I'm not an especially patient person, and I prefer to get as much done as possible with as little energy and time as possible. Designing six new games in a series - working from one core ruleset - is going to take a lot less time and effort on my part than designing six new games from scratch. The goal is to design more games more quickly and easily.
That's one reason why I take pains to make a core ruleset for a series as comprehensive as possible right from the get-go. I know some designers who take a more ad-hoc approach to designing a series, where each new game comes with new series rules, new unit types, retrofits for previous volumes so that they can be played with the new rules, notices to use these rules for this volume and not for that one, etc. That just feels clumsy to me - like the ruleset is a statue you've sculpted, only you glob on a clump of clay every couple of years - and it also strikes me as being pretty inefficient. I'd rather have a clean and consistent core ruleset that encompasses all but the most special of special cases right from the start. That requires more of me at the beginning of the process, when I'm doing the first game in the series, but it makes it that much easier for me when it's time to do another. In the long-run, it's saving me a lot of time and energy.
That's certainly been the case with the Shields & Swords II series of medieval battle games. Again, there's a special rule for this here, and some shenanigans with random deployments there, but the core rulebook has stayed the same since the series started with The Grunwald Swords. In fact, we had all the unit illustrations we thought we'd need done before the series began - even guys like horse archers, which wouldn't show up until Battles on the Ice, and crossbowmen, who still have yet to make an appearance! We use the same cast of characters in every game, which is very efficient budget-wise; we paid for all that upfront. The only unit illo we've had to commission since was a Viking for The Great Heathen Army, and that was only because we'd feel a little silly using our Veteran illustration with its big ol' red cross on the chest!
I knew going into the development of the Shields & Swords Ancients line that things would be a little different. We can kinda sorta get away with "generic" unit illustrations in the medieval games, but with Ancients each game's going to need its own illustrations every time, which is a pain in the wallet. You can't exactly have a bunch of hoplites dressed like centurions or vice-versa.
On top of that, however, there is the problem that you can't really have a bunch of hoplites fighting like centurions either. The time period we call "the middle ages" is sprawling and unwieldy, and there are major differences between battles in say the eighth century and the fourteenth. But within each distinct time period, there are broad similarities in the way Europeans bash and slash at each other, and so it's relatively easy to make small scenario-specific adjustments each time I do a new medieval game. But antiquity is something else altogether, encompassing a millennia and a half, and even if you carve that up into distinct time periods, you have vastly different armies fighting each other in vastly different ways. The idea of the Shields & Swords system - in either iteration - is to take a very broad strokes approach, but even within those parameters, cobbling together one core ruleset for all of it is a pretty tall order.
The first game is focusing on hoplite warfare in the Persian and Peloponnesian wars. I've already picked out my eight battles, I've read my Herodotus and Thucydides, I have my unit manifest ready. All that's been in place for a while, and yet I've been dragging my feet on completion. I realized that the only roadblock is that I hadn't quite figured out how the core rules are going to model chariots and elephants, concepts that have absolutely nothing to do with hoplite warfare in the Persian and Peloponnesian wars. It's ridiculous.
Like I said, I'm not an especially patient man, and I get frustrated when I feel like I'm not using my time and energy efficiently. It's why I try to front-load comprehensive system design, because it allows me to design more games more quickly and easily. But if I'm putting off doing the Battle of Marathon because I'm still figuring out what to do with imaginary charioteers, I'm not achieving that goal. It might be two or three years before I've found a way to model (even with the broadest of strokes) fifteen hundred years of battles fought in seemingly fifteen hundred different ways. And what I'll have to show for it is a core rulebook in that first volume with all sorts of rules for units that won't be seen until five or six games down the road. That hardly makes my job easier, and it doesn't do the gamer any favors either.
On top of that, I don't know how well the Shields & Swords Ancients line will sell. I like to say that the beauty of our model is that we can publish with impunity, but we're only inured from the financial fallout of a flop because we keep our art costs down. A series of games with one-off unit illos for each volume is a pretty pricey proposition - that changes the math, increases the risk. Probably it'll do just fine, but if it doesn't and we pull the plug after the first one? That's going to be a whole lot easier to swallow if I haven't put in all that time on war elephants and maniples that will never see the light of day.
And so, for the very same reason that I try to come up with one core series ruleset, I'm doing precisely the opposite. The Persian/Peloponnesian game will have one set of rules. The next game will have another, and the one after that another. Now, there will be considerable overlap between them - I feel strongly about using a square grid over hexes, and the familiar command chit kernel from the medieval games will be present and accounted for - but this will give me the flexibility to really consider how I'm going to model the multitudinous mayhems that sprawled across the ancient world. It will also make it easier to learn each game in turn, because then you're not trying to untangle unit types and rules that are irrelevant to what's currently on your table.
Just as importantly, it will let me design more games more quickly and easily.