The heroes and their circus arrive in Willowside, a swamp community in dire need of diversion from the relentless expansion of the blighted land known as the Welt. Willowside soon experiences a problem of a different sort, as dinosaur-riding xulgaths blockade the town. The heroes mount a defense of the village and learn what has brought the xulgaths to this isolated settlement. When the heroes break the siege, they learn that the xulgaths follow a monstrous ex-slave and have forsaken their directive to assault a nearby aeon tower. The tower's current occupants know more, and put the heroes on the path to reach the fifth and final aeon orb.

"Siege of the Dinosaurs" is a Pathfinder adventure for four 13th-level characters. This adventure continues the Extinction Curse Adventure Path, a six-part, monthly campaign in which the heroes lead a traveling circus as they unravel a plot to eradicate all life from the islands of the Inner Sea. This volume also includes a gazetteer of the town of Willowside and lore about the inhabitants of the Abyssal realm of Gluttondark, as well as new monsters and xulgath-themed rules.

Each monthly full-color softcover Pathfinder Adventure Path volume contains an in-depth adventure scenario, stats for several new monsters, and support articles meant to give Game Masters additional material to expand their campaign. Pathfinder Adventure Path volumes use the Open Game License and work with both the Pathfinder RPG and the world's oldest fantasy RPG.