Cullero is a city located in eastern Serkonos.
The city receives the bulk of the visitors to Serkonos and is a popular holiday location, as it has an abundance of sun in the warmer months. It is best known for its quality cigars. Cullero Cigars, a popular brand of cigars sold in Dunwall, originate here. Cullero also produces grapes.
Cullero was once a bog in the distant past. However, the Cullero Gap protected the area from the vagaries of the weather as well as the greatest plague of the eastern seas: pirates. Early traders set up outposts, eventually building piers to meet the ships far from shore. The seas often turned, eviscerating the boards and washing away the settlement, leaving the people to rebuild them over and over again. With time, Cullero became known for its military prowess. The Culleros earned a mean reputation on the seas, as mercantile bullies and smugglers, and many believed that they were not victims of piracy, but perpetrators of it. They traded heavily for iron, using it to shovel fill into the swamps and build a proper harbor.
Eventually, they decided not to rely on trade alone, and gathered their allies and sailed to Karnaca. There they lay siege, hoping for access to its iron resources, but unbeknownst to them, spies managed to slip through the blockades and appeal to the Lady of Saggunto, Ximena, for aid. However, when she tried to pursue, she found the guerrilla war to be too costly, and eventually, once the civilization in Saggunto collapsed, Karnaca and Cullero were left to fend for themselves, with neither city gaining in prominence at that time.
In the centuries that followed, Gristol grew militarily, leading to deserting Gristolian sailors to take up residence in Cullero, bringing in new military tactics and innovations. Unfortunately, this led to Cullero bearing the brunt of the War of Four Crowns. The Culleros welcomed the Duke and Duchess's coup in Karnaca that brought Serkonos into the fold of the Empire of the Isles, which saved countless Culleran lives. 
- Martha Cottings grew up in a small town outside Cullero.
- According to two Hatters, the black market is lucrative in Cullero.
- Cullero bears a great resemblance to the real-life Spanish city Cullera.
- There is an old Serkonan legend about the cliffs being haunted by what appears to be two ghost giants.
- Edithia Abele, a member of House Abele, is mentioned as being a steward of Cullero, and is said to have "swept the marshes of raiders, sacrificing her life for Serkonos".
- Cullero has its own notable law enforcement, referred to as the "Stewards of Cullero". They have relations to the Grand Guard in Karnaca as well as to the Greater Serkonan Guard, which presumably protects the rest of the isle. 
- Near Cullero, there is a place called Cullero Gap. A map found in Stilton Manor shows the tides of this place.