Siege Wall

A siege wall uses magical energy to fortify all external areas of a fortified building, such as walls, battlements, drawbridges, and gates. External surfaces to be protected must be contiguous.
The protective effects of the siege wall are compatible with BATTLESYSTEM rules. Creatures assaulting the protected building have their movement rates reduced by half when trying to scale the exterior surfaces (scaling ladders, etc.). Attackers suffer a -2 penalty to damage rolls for missile fire.
Damage or AD caused by war machines is reduced by 2 die levels (if normal damage is 1d12, 1d8 is rolled instead; if damage is 1dl10, 1d6 is rolled; ballista has AD8). Damage caused by crushing engines is rolled at -2 to the damage roll or ADs. Hits or hit points of crushing engines are reduced by half.
All enemies attacking a building protected by siege wall who enter an enclosed wall space are out of command unless they are in the line of sight of their commander, regardless of his control diameter.
All exterior areas of the fortification have their hit points or Hits doubled (see Hits of Building Features in BATTLESYSTEM rules).
The siege wall expires if the building is destroyed; it lasts a maximum of 24 hours.

[Tome of Magic]

Sphere: Creation, Guardian
(Alteration, Invocation)
Quest

Range: 480 yards
Components: V, S
Duration: Special
Casting Time: 1 turn
Area of Effect: One building
Saving Throw: None