Introduction
I'm GM, and my 7th-level players are planning to assault a fortress with an airship at night from a distance of roughly 100–2000 feet.
Background
The fortress is an academy for students of necromancy, so I have an excuse for some of its inhabitants to have some highly specialized, unexpected, and non-rules-as-written spells. I imagine most professors will be between level 7-10, while students will either be novices (~lv. 2-3) or adepts (lv. 4-5).
The encounter is supposed to be one of the most difficult encounters they've faced yet, if they decide to do a ground assault, but I don't think they will.
Question
How could the surprised, arcane inhabitants of the fortress defend it against such an assault—without resorting to catapults and ballistas? Most spells have prohibitively short ranges (e.g. Magic Missile is 100' + 10'/level), and the airship will be beyond this range for almost everything.
The only idea I have so far is that the defenders could summon some flying creature(s).
Ideal Answer
An ideal answer would have a brief list of types of spells, abilities and/or weapons that would be effective against an Eberron-type airship flying at a distance of 100-2000 ft, and would be reasonable for a diverse group of 2nd-10th level necromancers to have/use.
To clarify: a comprehensive listing of these things is entirely unreasonable and not in the spirit of this site. I'm looking for summarized lists: e.g. a certain family of spells that typically have long ranges, summoning certain kinds of flying creatures, some bows or crossbows (but only at shorter ranges), etc.
Further Details
I originally omitted details to make this more broadly applicable for any necromancy fortress getting attacked by an airship. Here is some information requested in the comments about my situation — and then some! 🙂
Fortress description: The fortress is roughly circular and is about 0.3 mi (500 m) in diameter. Thick stone walls surround it to a height of about 40 feet, and a half-dozen guards patrol its walls. There are a number of mostly-stone buildings within the compound.
Most of the party is actually under the fortress in a tunnel, having snuck in earlier in the night. One player was absent during the session, so we half-planned a solo adventure for him. The main party sent this player back to get the airship and its NPC crew. The directions were unclear: this player was either to create a diversion by dropping dozens of thunderstones on the fortress to allow the others to sneak away undetected, or to airlift the players out.
Airship Description: The airship is smaller and faster than typical Eberron airships because it was built for espionage missions. It is about 80' long and has a single, small swivel cannon at the bow. It currently has only 5 NPC crew members plus the player. I haven't determined an exact model.
Setting Description: The stone fortress is situated on a tiny northern island mostly covered in a thick coniferous forest. Pitch-black nights are rare in my interpretation of Eberron because there are several moons visible at any given time. It's an hour before dawn and there are several moons moons visible in various stages of waning. The weather is partly cloudy, and the faintly-golden Ring of Syberis is visible high toward the south.
For stealth, the player tried to smoke-screen the ship as much as possible, though it's difficult to hide the glowing ring of fire that moves the ship.
Best Answer
Since they have casters of levels 4 to 5, they have access to summoning spells, they have access to Fly (3rd level caster), they can fly closer to the airship and fire arrows and spells at the crew.
Spellcasting distance should not be a problem to those with access to Dimension Door (9th level caster) (with a range of at least 680 feet). So the 9th level casters should be able to quickly move around the battlefield.
Attacking at night is subject to concealment rules, even if using siege weaponry. I don't think the attackers have people with darkvision 100+ feet firing those siege weapons. So, even if the target's location and approximate distance is known, they have a 50% miss chance from concealment.
The simplest protection would be spells that works against ranged attacks that does not specifically mention siege weapons, like Protection from Arrows (2nd level caster). Being mostly necromancers, they should have no problem blinding (3rd level caster) those attackers with darkvision, or whoever is operating the siege weapons.
They could always target those aiming the siege weaponry, with damaging spells or force them to fall to their death bellow. Aiming a siege weapon before firing it requires a full-round action, so if you prevent them from using full-round actions, they cannot attack. Spells like Loathsome Veil or Babble (5th level caster) can do that.
A Tornado (if they happen to have a 13th level caster or a scroll) may cause siege combat to become impossible, and winds weaker than that should at least cause a small penalty (-4 for windstorms -8 for Hurricanes). But even a simple rain should cause a -4 penalty on perception checks.
At caster level 9, they have access to Summon Monster V, which includes things like the Babau (darkness and dispel magic), Barbazu (unhealable damage and reach attacks), Large Air Elementals (whirlwind), Kyton (fear effects, reach attacks and possibly can control the airship chains) and Xill (grappling and paralyzing attacks).
The SRD has a few of the necromancer-y type characters from the NPC Codex, which you can use as base for your school of necromancy: