Yes, the flower maidens are the most tile-efficient spammable population source atm if you only care about population sources. But since your point was caring about red attack primarily, the autumn vineyard still wins, because it actually provides an ok amount of red attack itself - which flower maidens do not.
Autumn Vineyard + 10 stormspires = ~32.5 squares for ~107 red attack boost on average = 3.29% red atk/tile (counting each stormspire as 6.5 attack boost for its average of 1% daunting tower fragments per day as well)
Flower Maidens + 3 stormspires = ~8 squares for ~19.5 red attack boost = 2.44% red atk/tile (and ~1.13% red def/tile)
And if you don't only care about red attack, then I'm not sure why you're obsessed with stormspires
Btw, the feta farm comparison = 17 atk / 6 tiles = 2.83% red atk/tile - and it does significantly more FP & Goods and throws in some treasury goods for good measure. Hence why I'm underwhelmed by stormspires in my high age world(s). If I want to stack more red attack, I'd lean to feta farms first.
Cider Garden, which will be reasonable to tile with even just 1 vineyard because you'll steadily accrue apple trees which will accelerate the rate at which you make cider gardens. 9 atk / 4 tiles = 2.25% red atk/tile + 13 / 4 = 3.25% red def/tile. Plus in Titan, 1.5 rogues/day each (and 1.5 light units - which in my testing are usable at low attrition in titan (i have a SAT diamond mine) - not great, but if they're free lol). And would be my target for an OP "easily free" combat-focused city over time. I do intend to have "some" of these myself - how many depending on needs in the space I end up leaving to be flexible.
The building I'm actually planning on anchoring on as my main building in my main (produced via flowering bougainvillea tree) :
Aegean Couple's Resort = 1.7% red atk/tile, 2.2% red def/tile, similar blue stats (less blue atk, more blue def) should the need for that increase (as it seems likely to), and finish special production items which will eventually be channeled at a chain building of some sort (once my city is satisfactorily constructed - until then the tree or another fragment producing building gets them) for a very-high daily FP income.