[Anaraxes]

The range for "present" at a ceremonial magical ritual isn't defined in RAW (AFAIK). The stadium example is probably the most extreme, but that's still under 100 m/yd radius. It's certainly not just being aware of the ritual, or LOS. So it's not obvious that the "doesn't work as long as the defenders stay put to oppose said ceremonial casting" assumption holds. Even if the defending mages were inside the range limit to count negatively on the ritual, it seems quite possible that erecting a barrier like Spell Wall or a NMZ between the hostile spectators and the ritual would block the magical effects of the hostility.

And beyond range, it also seems possible that the intent of the ritual designer matters -- if they didn't want you included, you're not, even if you're standing right there in their ritual chamber. Wouldn't stop a hostile mage from physically interfering, or even casting hostile spells. But if the hostile mage weren't invited into and made part of the ritual (as spectators are during, say, religious ceremonies, by being assigned necessary actions, chants, positions, etc), then those hostile mages are likely irrelevant, for magical success purposes of subtracting energy.

The big advantage the defenders have is time. Assuming the owner of the fortress can spend the money required, they'd have years to prepare, compared to an army that just shows up and starts the siege cold. A siege could, in theory, also last for years to match the defense prep, but sieges generally don't. On the other hand, if we're talking magical fortresses substituting for physical ones, you don't have those real-world constraints on low-tech sieges like the logistics feeding the besiegers and disease.

The prepared warlord also might just have mages show up with the products of their massive multi-year attack rituals Delayed / Linked / Hung. As long as his target has been fortifying, he's had his mages working on their ritual assault.