You Cannot Pass
A Brief Summary of Sorcerous Defenses
Spellcasters are classic enemies, but can be famously difficult to run correctly given the vast scope of options potentially available to them. Usually such concerns focus on running them effectively in a session, but a secondary dimension of similar import is their deployment of wards and precautions in advance of any conflict.
In ACKS, their ability to do so via magic is chiefly mediated by the limit of maintaining no more than one perpetual spell per caster level, at each level of spell slot known. Proactive sorcerers will naturally aim to maximize their use of such spells, ideally keeping at least one perpetual spell at each useful level (i.e. not 1st, unless custom spells are invented to provide benefit there).
To that end, I have listed all of the standard ACKS II spells of perpetual duration (with a parenthetical AD listing whether they are Arcane or Divine spells at that level), bolding those I believe to be particularly relevant for characters to maintain for the benefit of their sanctum.
2nd: illusory interior (A), magic lock (A/D), transform beast (D)
3rd: create chasm (A), ice sheet (A), incite madness (A), perpetual illumination/tenebrosity (AD), rune of warding (A/D)
4th: plant growth (A), wall of wood (A)
5th: forgetfulness (AD), plant growth (D), mirage (A), wall of stone (A)
6th: bath of the goddess (D), body swap (A), enslave humanoid (A), perpetual figment (A), transform other (A), wall of annihilation (A)
As you can see, arcane spellcasters have an overwhelmingly better selection, and it is on them that I shall focus.
At 2nd level, magic lock is the gold standard of defenses. It is strongly caster level dependent, so it makes sense to be cast by the master of a sanctum; illusory interior is useful to conceal spaces from cursory observation but is easily cast and maintained by an apprentice.
At 3rd level, rune of warding is similarly the outstanding candidate for defenses, as a universal defensive spell that allows all manner of other nasty offensive effects to be loaded (arcane casters have little reason to ever take the default damage option when they could instead charge it with a fireball, thunderbolt, or similar spell).
Perpetual illumination might be worth having an apprentice cast, but generally has too little advantage over mundane lighting to be worthwhile for a sorcerer’s defense compared to an extra rune of warding (for adventurers, of course, their utility is largely reversed). Perpetual tenebrosity, however, is circumstantially fantastic for monstrous spellcasters with echolocation or mechanoreception.
Ice sheet is worth mentioning as the lowest level option for a dismissible bridge or wall; it just requires water present. Combination with create water and/or weave water to provide such a source and create the requisite form can greatly expand the scope of such opportunities.
At 4th level, wall of wood is broadly superior to plant growth for area denial and as a dismissible bridge or wall when water is not conveniently present.
At 5th level, mirage is the clear winner, in combination with illusory interior gaining most of the utility of perpetual figment unless you specifically want illusions of creatures (which would be constrained to act on a preset loop).
Upon reaching 6th level spells, options abound. Enslave humanoid and transform other both provide access to powerful guardians for an arcanist’s lair; being invisible, wall of annihilation is a brutal trap.
So, broadly, when I build out a sorcerer’s lair, I assume he will have his level in magic locks and runes of warding. Both are simply too useful to pass up. At higher levels, mirage and some 6th level option are strong candidates, though I can understand why limited repertoire slots might not be available at such a point. It is hard to build useful 1st level options, but two notable additions present themselves and have proven a valuable addition to my repertoire:
Elemental Sentinel
Arcane/Eldritch 4
Drawing upon an elemental source within range, the caster animates it into the guise of a major elemental of the appropriate type, which acts on its own initiative each round. When manifested, the caster must task it with a single objective, such as guarding a location or hunting down a particular creature, stated aloud. The elemental will persist for one month and endeavor to complete that task (and can be given a new order if it does so). However, elementals are primal beings, unable to conceptualize complex orders (e.g. contingencies, nontrivial decision making, etc.), and will interpret their orders to bring reality into alignment with their elemental source (e.g. fire elementals incinerating their surroundings, earth elementals destroying stonework and draining lakes, etc.).Summoning (summons): summon a creature (85), up to 12 HD (x0.9), up to 1 special ability (x1), restricted to specific type (elemental) (x0.7), passively hostile (x0.8), controlled by speech/gesture (x0.8), draws on specific environmental source (elemental source to be animated) (x0.67), 1 turn casting time (x0.8), only cast 1/week (x0.8), 10’ range (x1), 1 month duration (x2.5), arcane/eldritch summoning (x1) = 36.74
Beguiling Kiss
Arcane/Eldritch 3With a kiss (requiring an attack throw against an unwilling target), the caster bewitches one living creature. If it has 2 HD or more, it can resist the effect with a Spells saving throw, which arcane or eldritch casters make at a -4 penalty. Otherwise, it is bewitched until dispelled or the caster chooses to stop sustaining the effect.
Enchantment: target bewitched for duration (40), touch range (x0.4), indefinite duration (x2.2), one creature of any HD (x1), only affects living (x1), save avoids if 2 or more HD (x0.75), save at -4 if arcane caster (x1.1), arcane/eldritch enchantment (x1) = 29.04
These two additions expand the scope available to casters. The first is not a perpetual effect, but gives them access to long term powerful summoned creatures as guardians. (Air and earth elementals in particular have useful mechanoreception, to detect invisible or otherwise stealthy intruders.) Summon manes, summon insect swarm, and various callings make good daily casts for a mage, but summon hellhounds has fierce competition at 3rd level and lesser hellhounds are barely better than war dogs as guardians (apart from the admittedly-useful ability to discern invisible, but personally I’d still favor a 3rd level variation on elemental guardian that manifests an 8 HD minor elemental). Summon shadow and summon invisible stalker are solid but better suited to skullduggery; summon ooze strikes me as unimpressive (for divine casters, however, summon insect plague is quite solid as a daily cast).
The second is simply the lower level pairing to enslave humanoid, which offers less total control but at merely 3rd level; it is sufficiently potent at that level to be competitive with the other excellent options already there, and provides an excuse to deploy living monsters as guards.
Some additional standard precautions include a daily casting of indiscernibility, to limit magical reconnaissance, as well as lead shielding around sensitive locations — coating a surface with a thin layer of lead costs only 0.25gp/square foot. It is thus quite cost effective to shield individual spaces, and even potentially entire fortresses (which could then have additional layers of internal shielding to prevent divinations from being particularly useful within them either). Lead will notably protect against greater clairaudiency, which indiscernibility importantly does not. Mundane secret doors are similarly useful, though still vulnerable to true seeing.
Introducing random elements into daily routines can also disrupt magical reconnaissance (inasmuch as a scrying spy still needs to know when to observe), and can foil augury and communion that can express only simple information and cannot foresee random results.
Routine checks with discern bewitchment and discern curse can also be beneficial to arrange, though nontrivial considering the limited availability of such spells. More practical may be to retain a contract killer or apprentice with a scroll of summon invisible stalker as a dead man’s switch, which will trigger on any loss of complex function (e.g. enslavement) as well as upon death.
The aforementioned war dogs are a cheap and loyal upgrade over normal soldiers; they have their own vulnerabilities, but access to 2+ HD guards is otherwise quite constrained and they seem exceedingly worthwhile to stiffen resistance.
Last, the Treasure Tome includes a number of excellent items of single or limited charges, which provide immunity to some effect such as poison, enchantment, or death spells. I would expect such items to feature as staples of the nobility’s standard allocation of items of power, along with other standouts such as chimes of warning or a ring of antimagic (excellent even on an NPC caster, to be worn passively and removed when needed for spellcasting or to move past sustained spells, as a defense against magical observation or surprise magical attacks).
Will a typical mage do all of the above? Likely not; he is merely human, and the demands of comfort and convenience leave holes in many defenses. However, our typical mage may well take a scattering of the above precautions, and truly wary, paranoid examples will not be so terribly uncommon — particularly if they expect adventurers to come knocking. Perhaps some of these elementary tricks can help keep them alive a little longer, and give the ever-beleaguered Judge a starting place in building out their sanctums.