Invisibility's utility for infiltration can be thwarted by closed doors and guard dogs.
That was my suggestion.
celebrim said:
Successful castle defense is going to have dedicated systems for responding to invisible threats. The front line of this defense is probably going to be trained creatures with the scent ability...
Guard dogs are an available low level solution. Additional hardening of the access to gate houses and internal checkpoints using techniques similar to those used to control the flow of prisoners help but also increase the time required to shift defenders to defend a breach. These technique are more along the lines of 'make do with what we have' rather than really good solutions. Once you have a serious security breach into internal living spaces, having gaurd dogs and checkpoints everywhere reduces live ability to the point that it no longer seems appropriate simulation. Large doors that see high traffic are breached fairly easily except during a seige, and the whole point of the invisible approach is to try to achieve surprise. Truly succesful defense then involves having invisibility countermeasures and alarms at major corridor intersections and important gates, and an anti-infiltrator task force (guard dogs, low level wizards with glitterdust, fighters trained in blindfighting, high-ish level NPC's, etc.) ready to respond to threats.
The main thrust of my comments is that if you want to minimize the impact invisibility has on defensive design, that you need to make those magical countermeasures relatively cheap, long duration, and available at relatively low caster level. I think there is some justification for this. Magic Mouth provides a template for what can be done in terms of duration (triggered spell that is permanent until discharged at 2nd level, 1st level for Bards), and Faithful Hound provides a fairly high level example as a cap on how effective a lower level countermeasure may be. Alarm doesn't have the nice 'permenent until discharged' or 'trigger' effects, but it isn't fooled by invisible creatures either. See invisibility is a 2nd level spell, so detecting invisible creatures shouldn't be particularly high level. Glitterdust provides an effective countermeasure to invisibility as a 2nd level spell. Put this all together and its suggests that spell based invisibility counter measures with all the desired properties ought not to be more than 3rd or 4th level.
Permenent until discharged 'Alarm': This could be a 2nd level spell, and while you couldn't defend trafficed corridors with it, its an effective countermeasure in ventilation, pipes, and sewers.
'Magic Mouth' with 'see invisibility' feature, triggered to go off when an invisible creature comes in range: Probably 3rd level, and is an excellent counter measure for internal intersections and large halls where you can station sentries but doors aren't practical.
'Glitterdust' with 'permenent until discharged' duration: This would be about 4th level, and is an excellect countermeasure at large entry points where at least some of the gaurds are shielded behind fighting positions. Can also be combined with any of the above to summon additional help, so a castle which has a 7th level mage dedicated to its defense (which even most 'grim and gritty' sorts of campaigns usually won't balk at too much) can still make a major hastle without spending alot of XP.
'Arcane Lock' with specified trigger: At 3rd level you could have an improved arcane lock, where the pass trigger can be specified by the caster to be something other than 'self'. For example, the tigger might be, "Pass only humans wearing this uniform and holy symbol, and who speak the word XYZZY". This provides excellent access control while still allowing freedom of movement for the inhabitants.
Faithful Hound: Available at 5th level, but doesn't have the 'cast it and forget about it' feature that so important in a defensive spell. High level characters might have this spell available (probably on scrolls) to guard leaders, wells, and other critical points of the defence. Also, at 9th level and higher, you can start making at least some of the above effects permenent, which has a high up front cost but means you don't generally have to replace the spell once it is triggered.
After that, consideration must be made to locking down a castle from teleportation attack...