[Micah]

The key here is that defense should be stronger than offense. It is largely immaterial to a balance discussion if the last 4 flames from the sky get through in the example provided (one with a reasonable amount of high-value targets), the Flames caster is already ahead economically as soon as the first one gets through. I don't see how it is in any way "unbalanced" to have the other 4 bounce off of a second dome, the dome caster has still lost, just not by as much as they could have. Stacking a ton of valuable stuff into a single province just isn't a good idea, stackable domes or no, since there comes a point when artillery spells will still pay off on average. Spreading out forces and popping out domes everywhere comes with its own set of problems, since it drastically limits your defensive mobility, and of course every small outpost would cost more gems to set up.

Multiple types of domes stacking is a different issue entirely, and really doesn't interact with the multiple air dome issue very meaningfully. (The only way it comes into play is if someone was determined enough to launch enough artillery strikes to knock down the single air dome and continue casting through the now-somewhat-lowered defenses. With an astral and frost dome in the same province that still knocks out 3 out of 4 spells. Not a good ratio for single target spells, and pretty impractical for anything expensive, since you'd need about 20 casts to get through the air dome on average, assuming proper ordering and interaction with the domes.)