Castles and Sieges

(This article is mainly for GM Purposes)

Keep in mind the following things for any and all buildings. These spells are all permanent:
Arcane lock (2nd level)
Continual flame (2nd level)
Immovable object (upcast to 6th level)
Glyph of warding (3rd level)
Forbiddance (6th level)
Hallow (5th level)

Consider for sieges:

Defenders:

Spell: Control weather to obscure archer sightlines, or turn the temperature up to 40°C to make everybody wearing any form of armor pass out. (Unless they have resistance to fire damage)
Walls would have machiculations, and have roofs to cover from offensive spells and attacks from above (hordings)
The castle would have its own mages to lob AOE spells into any crowds of attackers.
The walls of the castle could also be magically reinforced (adamantine-coated metal supports, enchanted stones in the center, enchanted stones covered by mortar or something) making them very resistant to spells such as shatter.
Defenders would send out strike teams and assasins to take out important members of the attacking army, such as commanding officers or mages. They might also sabotage the attackers.

Defenders might also have a teleportation circle (lvl 5) or antimagic-fields inside to assist them.

The attackers

would have spells to tear down the castle walls. (Shatter, earthquake)
Or to just kill the defenders inside their fortifications (stinking cloud, sickening radiance, cloudkill etc)
They would have teleportation spells (misty step), or summoning spells (summon elemental) to bring their troops up onto the walls if any opening presents itself.
Attackers would try to send assassins after important defenders, such as the commanding officer or the defending wizard, or sabotage the defenders.