Siege is not in the current map series (Keepers) for the Atlas of Worlds. In permanent leagues, an existing Siege Map can be used in the Map Device, but it cannot be influenced by the Eldritch Horrors or witnessed by the Maven. There is no bonus objective for this map. It can be converted to a map from the current series by selling it to a vendor.
The following Maps have been added back to the Atlas: Arid Lake, Armoury, Barrows, Beach, Belfry, Cells, Chateau, Conservatory, Core, Crystal Ore, Desert Spring, Dig, Excavation, Fields, Flooded Mine, Fungal Hollow, Geode, Jungle Valley, Lighthouse, Malformation, Marshes, Pit, Racecourse, Ramparts, Relic Chambers, Scriptorium, Siege, Stagnation, Tower, Tropical Island, Waste Pool, and Wharf.
The following Maps have been removed from the Atlas: Alleyways, Barrows, Bog, Chateau, Core, Desert, Foundry, Fungal Hollow, Lava Lake, Leyline, Lighthouse, Mud Geyser, Park, Pit, Plateau, Primordial Blocks, Racecourse, Reef, Relic Chambers, Sepulchre, Siege, Spider Lair, Sulphur Vents, Sunken City, Terrace, Tropical Island, Waterways, and Wharf.