I think I'm missing the illusion. Who is being deceived about what?
Clearly, it's a good illusion, if even you are missing it!
Maybe a hypothetical non-illusory treadmill would help:
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Bob, a veteran (1) fighter in service to the local lord(9), Bert, an apprentice (1) thief in service to the guildmaster (10), Robin a sorceress's (9) apprentice (1) who just mastered Sleep, and Robere a novitiate (1) sent by the high priest (9), are brought together to perform missions to protect their town. First they route a gang of bandits(1HD), saving a merchant(expert3) who gives them a reward(100gp) that they use to buy Cure Light Wounds potions when they drop Robere off at the temple on the way home. They all train up to (2). Next they head out to clear a haunted crypt, facing zombies (2HD), which Bob recognizes as casualties of that battle he was in (veteran, remember), meaning, the DM explains through high-INT Robin, that they must have been recently animated by an evil Necromancer(10). Sleep and backstab do nothing, so Robin casts magic missiles and Bert sticks to his sling while Bob fights and Robere Turns their way to victory. Bert gets them past a fiendish(tm) trap (built by Ginko the Fiendish, kobold expert5) to confront the necromancer(10), who casts Heightened Greater Improved Sleep(5) on them and escapes. They gather up what treasure the necromancer didn't want - plenty of armor & weapons for Bob, Bert & Robere - return to town, on the way, they save Bob's older brother, Rob(commoner4) from orc raiders (warrior3). They report back to their respective mentors and train up to (3), Robin learns Lesser Improved Sleep, and can now cast two magic missiles, Robere now casts Cure Moderate Wounds.
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Bob, a fighter (3) in service to the local lord(11), Bert, a thief (3) in service to the guildmaster (12), Robin a wizards (11) apprentice (3) who just mastered Lesser Improved Sleep, and Robere a cleric (3) sent by the high priest (11), are brought together to perform missions to protect their town. First they route a gang of Bandits(the unofficial NPC Class 3), saving a merchant(expert5) who gives them a reward(200gp) that they use to buy Cure Moderate Wounds potions when they drop Robere off at the temple on the way home. They all train up to (4). Next they head out to clear a haunted crypt, facing zombies (2HD+2 levels of Barbarian), which Bob recognizes as casualties of a recent skirmish, meaning, the DM explains through high-INT Robin, that they must have been recently animated by an evil wizard(12). Sleep and backstab do nothing, so Robin casts magic missiles and Bert sticks to his sling while Bob fights and Robere Turns their way to victory. Bert gets them past a fiendish(tm) trap (built by Ginko the Fiendish, kobold expert7) to confront the necromancer(12), who casts Lesser Death-Like Sleep(6) on them and escapes. They gather up what treasure the necromancer didn't want - plenty of armor & weapons for Bob, Bert & Robere - return to town, on the way, they save Bob's older brother, Rob(commoner6) from orc raiders (warrior5). They report back to their respective mentors and train up to (5), Robin learns nominal Improved Sleep, and can now cast 3 magic missiles, Robere now casts Cure Serious Wounds.<br>
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Bob, a fighter (5) in service to the local lord(13), Bert, a thief (5) in service to the guildmaster (14), Robin a wizards (13) apprentice (5) who just mastered Lesser Improved Sleep, and Robere a cleric (5) sent by the high priest (13), are brought together to perform missions to protect their town. First they route a gang of Bandits(the unofficial NPC Class 5), saving a merchant(expert7) who gives them a reward(500gp) that they use to buy Cure Seroous Wounds potions when they drop Robere off at the temple on the way home. They all train up to (6). Next they head out to clear a haunted crypt, facing zombies (2HD+4 levels of Barbarian), which Bob recognizes as casualties of a recent skirmish, meaning, the DM explains through high-INT Robin, that they must have been recently animated by an evil wizard(14). Sleep and backstab do nothing, so Robin casts magic missiles and Bert sticks to his sling while Bob fights and Robere Turns their way to victory. Bert gets them past a fiendish(tm) trap (built by Ginko the Fiendish, kobold expert9) to confront the necromancer(14), who casts nominal Death-Like Sleep(7) on them and escapes. They gather up what treasure the necromancer didn't want - plenty of armor & weapons for Bob, Bert & Robere - return to town, on the way, they save Bob's older brother, Rob(commoner8) from orc raiders (warrior7). They report back to their respective mentors and train up to (7), Robin learns Greater Improved Sleep, and can now cast 4 magic missiles, Robere now casts Cure Critical Wounds.<br>
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Bob, a fighter (7) in service to the local lord(15), Bert, a thief (7) in service to the guildmaster (16), Robin a wizards (15) apprentice (7) who just mastered Lesser Improved Sleep, and Robere a cleric (7) sent by the high priest (15), are brought together to perform missions to protect their town. First they route a gang of Bandits(the unofficial NPC Class 7), saving a merchant(expert9) who gives them a reward(1000gp) that they use to buy Cure Critical Wounds potions when they drop Robere off at the temple on the way home. They all train up to (8). Next they head out to clear a haunted crypt, facing zombies (2HD+6 levels of Barbarian), which Bob recognizes as casualties of a recent skirmish, meaning, the DM explains through high-INT Robin, that they must have been recently animated by an evil Mage(16). Sleep and backstab do nothing, so Robin casts magic missiles and Bert sticks to his sling while Bob fights and Robere Turns their way to victory. Bert gets them past a fiendish(tm) trap (built by Ginko the Fiendish, kobold expert11) to confront the Mage (16), who casts Young Dr. Kildaire's Medically-induced Coma(8) on them and escapes. They gather up what treasure the necromancer didn't want - plenty of armor & weapons for Bob, Bert & Robere - return to town, on the way, they save Bob's older brother, Rob(commoner10) from orc raiders (warrior9). They report back to their respective mentors and train up to (9), Robin learns Heightened Greater Improved Sleep(5), and can now cast 5 magic missiles, Robere now casts Cure Emergent Trauma.<br>
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With a level tolerance of (say) +/-3, I'm not sure it's a ton.
I suspect it'd be around 5, ok maybe 7:
1) Skill Challenge: the foe is so overwhelming all you can hope to do is avoid fighting it.
2) Solo: The party is capable enough to take on the same foe if they work as a team, and catch it alone.
3) Elite: the party has progressed to the point that they're individually a match of the same creature.
4) Standard: the party is so practiced at defeating the foe that consistently win at even odds.
5)Minion: Any member of the party, can, with a bit of luck, dispatch one of these same creatures in a moment.
6)Swarm: only mass waves of dozens or hundreds are still a threat.
7)Skill Challenge: Though the party could lay waste the entire population, there's something more they need from them than can be gained by slaughter or intimidation, alone.
