Guardian conjured onto a massive door
A Guardian is a round creature with many eyes, and appears to have been melded to a closed entrance. Guardians are summoned by high-level divine classes, such as clerics, druids, paladins, etc. These classes can bring a Guardian into being with either a summon monster or summon nature’s ally spell, using the Craft Wondrous Item feat to manifest the seal. Guardians are typically summoned to guard great goods or evils, such as powerful angels, demons or devils, or benign or malign artifacts, benevolent or malevolent high-level spellcasters, and so forth. Rarely, powerful neutrals are imprisoned.
Logistics require that the guarded objects or creatures be inside a reasonably enclosed space such as a cave, room or other physical cage. A thin crack under a door notwithstanding, the entire enclosed area is surrounded by divine power supplied directly by a deity of the summoner. No amount of mortal force or magic can pass through a door, window, wall, floor, or ceiling so protected. The godly power is channeled through the Guardian, which, if destroyed, removes the field and allows access both from within and without.
COMBAT
Using telepathy, the Guardian first warns away all creatures, succinctly explaining that "no creature may pass this way." It further warns that it must destroy anyone who does not leave immediately and uses its fear thought magic as additional motivation. It perceives any resistance to its warnings as a hostile act and responds with immediate force. If the seal has a weakness, it is its immobility. Its many eyes serve only to scan the horizon for intruders.
Gaining access to the area beyond (breaking the divine enchantment) requires defeating the Guardian in combat. Any creature that actively helps defeat a Guardian is automatically inflicted with bestow progressive curse (new spell).
Psionics (Sp): Each round a Guardian can use its thoughts to direct a spell-like ability, including aid, charm monster, command, detect thoughts, dismissal, dispel magic, dissolution (new), fear, flesh to stone, greater torpor (new), harm, hold monster, holy aura, holy word, mage’s disjunction, magic missile, mass heal, power word blind, power word stun, prismatic wall, telekinesis, telepathy, tongues and true seeing. Any of these spell-like powers can target creatures or areas within line-of-sight, even if the spell normally requires touch (treat as a ranged touch attack).