Abjuration / Common Permanent Spells / Magical Defense

Castles & Forts

Castle defences against magic:
Air cleaners: sucks in dangerous gasses and spits out air. Advantage on saves turn one, no gas turn 2.
Spell Slits: The walls are enchanted so magic can only pass through arrow slits. It makes it hard for attacking mages to hit even with AOE spells.
Enchanted ground: enchanted against being watered down or moved
Revealing Flames, invisible things are seen in the light of these flames
Anti Air Towers: Towers equipped with magic cannons. Thes ecannon can dispel flight, knock beings prone or have it easier to hit flying targets.
Storm cloud: Storm cloud that resides over the castle. It strikes anything flying within range and makes it hards to fly near it.
Silenced grounds. The area around the castle is completely silent. Verbal spells are therefor impossible to cast near the castle.
Teleportation magnet: A room in which anybody trying to teleport inside arrives in. This makes it easy to now when and who enters.
Will enhancer: advantage on saving throws against being charmed
Bell of detect magic: Toll whenever it detects a spell being cast. May chime differently depending on school of magic.

Strategy's to attack
Teleport bomb. It may destroy teleportation magnets

City Defense

There are many ways to defend a city with magic and while most city have soem sort of magic defense no one has even close to all of them.

Elementals

Elementals summoned to aid the city.

Golems

Statues coming to life
Warrior Golems Your standard golem.
Clay golems was first created by handy gnomes from The Teeth. Their use was spread to the southern human settlements from there.
Stone golems, given energy by magic crystals, was first created by the dwarfes to combat the abominations. They were later implemented as part of the dwarfen city defense. The dwarfen knowledge was borrowed by Brend during The Eternal War. Since then 10 large stone statues has stood outside the outer walls waiting for an army do destroy. Each of these statues looks like an armor clad warrior with mace and shield, all apinted in the colours of the city, blue and red.
Mage Hunter Golems Golems enchanted with detect magic to be easier be able to find mages in enemy armies. Generally buried beneath grioud or hidden in plain sight. Unactivated during peace times and while active during war sleeping until they percive magic being cast within its sight. Upon awakening trying to kill anyone it believes to be a mage. Generally are enchanted with defenses against magic, coutnerspell and offensive spells.

Elemental Walls

Gas & Fog

Anti Magic Warmachines

Catapults and trebuchets armed with ammuntion that casts dispel magic on anything within range. Used to dispel flight magic against airborn enemies before they come on top of a city.

Throne Hall and other Rooms of Interest

VIP Specific people like a royals and council members may by blood, ritual or owning of a specific item, be excluded from negative effects. Those people of interest often also have some special spells that may only be cast on them.
Dispel Magic Dispels any spell of 3rd level or lower. Does not affect higher level spells.
Glimmer of Detect Magic Any spell, magic effect or magic item in the room glimmers. This include any divination spell cast to spy in the room.
Defense Against Inbound Teleport Any teleport into the room, including summoning of living beings, are defended against. Dispel Magic (+3 to rolls) is cast against any spell of that sort.
Death Ward Combo All humanoids in the room have the effect of Death Ward upon them. After a creature is saved from dying from this effect it stays active for some seconds after which the magic is drained and has to be replenished.
When the effect actives chained spells are activated to protect people in the room. The chained spells differs but they are all meant to protect the people in the room and slow down the action. A common combo of spells would be: Healing Word (to heal up those who have been hurt), Lesser Restoration and Hold Person on each person (excluding VIP) in the room.
Mage Armor VIP may have Mage Armor cast upon them when in need as a reaction.
Escape VIP may have the ability in emergency teleport to a safehouse.

Holy Buildings

Glimmer of Detect Magic Any spell, magic effect or magic item in the room glimmers.
Blessing of the Shrine Any follower of the religion has the following effects while in the holy building: *Bless. *Effective heals, healing magic cast always heal the maximum amount. *Calmness.
Banishment A follower of the religion can cast banishment upon a creature within the building. The banishment is channeled by the building itself, can only be done once per day, and only activates if it is in line with the religion.

City Entrance and Gates

Dispel Magic Dispels any spell of 3rd level or lower. Does not affect higher level spells.
Glimmer of Detect Magic Any spell, magic effect or magic item that passes through the gate glimmers.