Long ago, a now-lost civilization carved several platforms on this natural upthrust tower of stone. Steep, straight ramps lead up to each oddly level platform. All traces of the past civilization’s activity are lost except for a perfectly circular depression on the top platform, in which faint runes can just be discerned.
Recently, a demon cult learned that the circle can be used to summon demons. They climbed Demons’ Tower and manned it with a dozen cultists and their minions: a small pack of gnolls.
When the PCs arrive, six cultists perform a ritual at the top of the tower (area 1). The party has a limited amount of time–say, 20 combat rounds, or two minutes–to reach the top and disrupt the cultists before the ritual is completed, a portal to the Nine Hells appears, and a new demon charges through it every round.
The next platform down (area 2) contains four cultists, all of whom will cast spells as soon as enemies enter the entrance zone at the bottom.
The platform below that (area 3) contains two more cultists and two gnolls armed with bows. All will used ranged attacks against enemies in the entrance zone (area 4).
The entrance zone (area 4) contains four gnolls armed with glaives; two will charge into melee and the others will stay behind the barricade and defend the first ramp. Thanks to the glaives’ reach, the defenders behind the barricade can still attack enemies who are on the other side of the barricade. A creature can climb over the barricade with a successful DC 15 Acrobatics (Dexterity) or Athletics (Strength) check.