Until the early 1900s, Yeouido was farming land, [oh really?] but in 1916 during Japan's colonial rule of Korea (1910-1945), the Japanese army built an airstrip there.There's some interesting information there (even if Yeouido was not used as farmland), but this is not entirely correct:
In May 1920, two Italian airmen making a cross-Eurasian flight arrived at the airfield before heading for Tokyo, and were greeted by tens of thousands of Koreans in a welcoming ceremony to see the new means of transportation. Two years later, Korea's first pilot Ahn Chang-nam who studied in Japan flew from Japan to Korea and was met with cheers from Koreans at the airfield.
In April 1929, the airstrip was expanded and was upgraded to an official airport as part of Japan's plan to make Yeouido a stopover for flights between Japan and Manchuria. As an official airport, Yeouido was equipped with basic facilities, including staff offices, telephones, anemoscope, anemometer and a 231-square-meter terminal. The official opening ceremony was held on Sept. 24 that year, the same day as another airport was opened in Ulsan in the southeast of the peninsula. In addition the Japanese colonial authorities built airports in Daegu, Pyongyang, Sineuiju, Cheongjin and Hamheung.
The airport was turned into a plaza where mass gatherings were held, and became a bicycle park at weekends. In 1999, the asphalt was removed, grass was planted and the site was renamed Yeouido Park.I'm assuming, with the reference to asphalt, "airport" above means "runway". If that's the case, it might be worth looking at some photos of the runway before 1971. Here's a shot of it in 1952, when it was used as an airport during the Korean War (more photos by this photographer of Yeouido, Seoul and Gimpo Airport during the Korean War can be found here).
Here are two shots taken in 1968, when the levee was built around the island to prepare it for development. The first looks west.
This one looks southeast.
If these aren't clear enough, here's a map from either the colonial era or in the years after independence showing the location of the runway.
Yeouido Park runs almost perpendicular to it, so I think we can be pretty sure it was never the runway.