Seoul is the capital and largest city of South Korea.
U-GIN Genetic Research Facility
Ultron, Scarlet Witch, and Quicksilver later arrived in Seoul where he arrived at the lab of U-GIN Genetic Research Facility and confronted Helen Cho. Ultron informed her that he intended to use the Regeneration Cradle to create the perfect synthetic body for Ultron, made from Vibranium. When Cho resisted Ultron used the Scepter to take over control of her mind and force her to do his bidding.
As the synthetic body was built inside the Regeneration Cradle, and was given a Vibranium composition on Ultron's request, thus making it more durable, Ultron revealed to the Maximoff twins the power contained inside the Scepter: an Infinity Stone, with which Ultron would integrate with his new synthetic body and adopt god-like powers. Cho then plugged Ultron into the machine to upload his base consciousness into the being. When Scarlet Witch attempted to read Ultron's mind through the synthetic body, the being allowed her to discover that Ultron's actual plan was to cause a global catastrophe that would usher in a new age of Earth's evolution.
Ultron attempted to defend his actions, noting that there was no room for the weak in the world, however, doubting Ultron's cause, Scarlet Witch removed Ultron's mind-control over Cho in secret to allow her to sabotage the upload. Ultron sensed that the Avengers had arrived in Seoul to stop his plans and ordered his soldiers to evacuate. However Cho used the opportunity to stop the upload, in response Ultron fired a blast of energy at her which stunned her, in the chaos the Maximoffs escaped and Ultron was left with no choice but to move the Cradle onto a truck and escape.
Hawkeye managed to locate Ultron in a U-GIN Truck. Hawkeye suggested shooting it, but Captain America pointed out the possibility that the Infinity Stone inside Ultron's synthetic body could destroy the city. Rogers jumped on the truck, interrupting Ultron's download. Rogers decided to keep Ultron busy so that Hawkeye and Black Widow could retrieve the body. Rogers and Ultron fought, with Rogers doing his best to hold his own. Rogers lost his shield. Following the battle onboard a motorcycle, Black Widow was able to retrieve it for him. The fight eventually spilled into a moving train.
Meanwhile, Romanoff entered the truck; however, the Ultron Sentries lifted the truck into the air. On the train, Ultron almost defeated Rogers when Scarlet Witch and Quicksilver came to Rogers' aid. Ultron begged the twins not to turn against him, but Scarlet Witch pointed out that they had no choice. Ultron killed the conductor of the train and fled. This caused the train to speed out of control. Rogers had Quicksilver move civilians out of the way while Scarlet Witch used her powers to successfully stop the train. Romanoff managed to roll the Regeneration Cradle into the Quinjet in midair, but she was kidnapped by Ultron in the process.
Seoul was among the cities noted by WHiH News anchor Christine Everhart to be still "on the mend" as a result of the incidents involving the Avengers, in part due to their tactical response, as late as May 3, 2016 when the issue of the Avengers' management and responsibility was being discussed.
To be added
This section requires expansion