For some of the most fascinating architecture and history right near Hongdae, the Jeoldusan Martyrs' Park is the secret treasure of Seodaemun-gu. Statues tall and small are made to commemorate these martyrs and Jeoldusan Martyrs' Museum is open during the day (10am- 5pm [Apr - Nov] / 10am 4:30pm [Dec - Mar]) showing the lives (and tortured deaths) of these martyrs. Only a few meters away is also the Yanghwajin Foreigners' Cemetery, where many foreign missionaries are buried. While daytime hours for the museum apply, it's possible to go the Martyrs' Park at night. Though one should be respectful of those praying in the dark, the lit-up Votive candles glowing only a few steps away from the Han River are beautiful for all, not just the religious.