Hell Café is a trendy place with amazing service. they have great offers and discounts. You can use their Wi-Fi and listen to classical music while pulling out your work.

Rocketeer Coworking space and a café at a same time. It has meeting rooms and is open all day. You also have he option of giga Wi-Fi, lockers, bike parking, bar and food. The average internet speed is 100mbps.

Olleh avenue is a store where you can get prepaid sim cards, their staff is friendly and speak in English too.

WeWork and Hive Arena are the most popular co-working spaces here. Although WeWork has 3 posh large spaces here, Hive is a cheaper option and close to the metro. I really liked the metro system which is extensive and everyone knows English! It is worth taking a trip to Jeju island from here.

Hive Arena co-working space has a great community with mix of local and international people from various field (tech, food, fashion, gaming etc). This place is also popular for meetups which take place regularly.

Tom N Tom's Coffee is great WiFi cafe with pretty good WiFi speed (around 60mbps upload and download) which is great for Skype and general laptop work. Service was Fast and Friendly and it's open 24 hours.