Complex sentence: It's located in the district of Yangcheon-gu.The building was the tallest in the country since 2003 till its been surpassed by Tower Palace Three Tower in 2004.
Split into simpler sentences: It's located in the district of Yangcheon-gu. At the time of its completion, the building was the tallest in the country but was surpassed by Tower Palace Three Tower in 2004.