Another important thing to remember, is that normal units can be mass produced. You speak of high tier national units as if they're really special and awesome. While some of them with level requirements aren't so easy to get, like immortals, high priests et al, others, like paladins, rangers, champions, eidolons, berserkers, musketmen, longbowmen, crossbowmen, and such things, can be mass produced. It's not unfeasible to have several cities with the production capacity to churn out a high level unit every single turn.
Mass produced to a maximum of 4, which isn't a terribly massive number. Even if it's one per turn, you're only going to get 4 of them.
The archos spiders are limited to "whenever the hell the game feels like giving you them", and can't be produced on demand. Now obviously, they're not intended to be your rank and file soldiers. But the spiders are rarer, and special.
Slightly less rare in the redux - Dark Wealds have had their bonus increased and baby spiders count half as much as they used to. I've also dropped the nest population requirements for each stage slightly (4 to spawn, 8 for giant, 12 for strong)
And you're coming up with a lot of cool mechanics to improve them here. But if at their ultimate potential, they're not going to be able to stand up to high tier units, what is the point of them?
3/4 or more of the game isn't played with high tier units. That is the period for which they are designed and are useful. By the end game, you should have Mother, Haruspex and Spiderkin - all of which are potent enough, especially when backed up by a potentially large number of Strength 12 (or 15.4 if they've gone for Greater Sword Spider) units.
As far as I can see, your main argument is that the spiders might be too powerful because the archos don't have to invest production in building them, and can get them for free. But I feel this is counteracted by the lack of control in when you get them, and an inability to mass produce them.
Yup - that's the balancing factor. Just remains to be seen if it's possible to get too many, too quickly despite that restriction. They are free, hidden nationality, potentially invisible (even if recon isn't that rare) and require nothing to obtain except growing your capital. Even the method for improving them costs nothing - it uses only the free units themselves. Anything you gain from them is a bonus to what you have available anyway.
Additionally you have less building requirements than most civs which means more hammers to be spent on units earlier in the game, allowing you to land grab effectively at the beginning.
WarKirby said:
If you kill a paladin, it's a simple matter for the enemy to upgrade a priest, or just build another paladin. I have actually seen this happen. An AI player with hundreds of priests, just upgraded them into new paladins as fast as the old ones fell. If they kill one of your spiders, you have no immediate recourse for replacing it, except time and luck. I feel that justifies them being fairly exceptional in strength, and that they should be capable of standing up to almost anything when properly promoted (heroes excluded, generally).
If you're at a stage of the game where Paladins/Eidolons are available, feel free to build some yourself. Or Recluses, or Spiderkin, or Chosen, or Eternals, or anything else. You'll still get the spiders, so you'll be that many free units ahead of the opponent.
The chance of getting a new spider each turn is [Nest Population + (Number of Cities*2) + (Number of Dark Wealds*4) - (2*Number Giant Spiders + Number of Baby Spiders)]. With a nest population of 12 and 8 cities total, you're looking at a 36% chance per turn if you currently have no spiders, or 8% if you already have 10 Giant Spiders. Every Dark Weald you own also negates the probability-cost of 2 Giant Spiders, so 8 cities each with a Dark Weald would give a total of (12 + 8*2 + 8*4) for 60% chance if you currently have none, and a maximum of 30 Giant Spiders.
WarKirby said:
Remember also, that nothing is invincible. Even a str 30 unit can be taken down with relative ease, if the enemy spams enough champions at it. Strength is a primary determining factor in single combat. But being mobbed by large groups can easily make strength irrelevant, beyond maybe incrementing the number of cannon fodder units the enemy needs to expend.
Spamming 120

units with a moderate tech cost to deal with a free unit with no tech cost is a problem. After you've used 4 champions to kill the high strength unit, you still have to deal with the 4 champions that your opponent still has (as there is no penalty to their ability to create standard units due to their free unit).
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Things are done for now in anycase and the changes are committed. The Archos weren't weak as a civ beforehand when the spiders were a flavour mechanic rather than something you'd want to concentrate on. Now it's very much worthwhile to build up the Broodchamber for at least the first 3 tiers of the game, with them still having some use during Tier 4.