To be honest, this world heritage site has one of the lowest overall scores within our community and I think I know why. I believe this is because instead of Baekje's rich history being centralized in one place (like Gyeongju), you have Baekje's three former capitals. Thus the value is somewhat diluted from a visitor experience. Furthermore, if any of the Baekje sites in Seoul are inscribed as an extension, what then? Lastly, its perplexing that Korea did not include other Baekje treasures in the region, instead only focusing on the fomer capitals. I can assure you Baekje has other treasures that rival Silla that were not included in this WHS.
Gongju & Buyeo
One of the most important sites in this serial property is the Tomb of King Muryeong. The famous tomb, which was discovered intact, became one of Korea's most famous archeological discoveries. The tomb included items from Southeast Asia, China, and Japan. The interior of the tomb can only be visited by an onsite replica. Like Kaesong and Gyeongju, this WHS is made up of mostly tombs, fortresses, and temples. The fortress in Gongju and Buyeo are both pleasant walks, but have been partially rebuilt in later historical periods. Of all the Baekje components I explored, only Naesong City Wall remains unvisited, which I hope to correct in the future.
Iksan
While Gongju and Buyeo are easy to visit, it gets more complicated in Iksan. First and foremost if you visit Iksan hopefully it's by car, as these sites are spread out and only few of them are accessible by bus. I visited Mireuksa temple site and the Wanggung-ri Palace site. These historic areas really feel like archeological sites. There are few remains, but fascinating nonetheless. The famous and enormous Mireuksa 5-story pagoda (originally 7-story) was taken down and then painstakingly resembled over many years. Mireuksa pagoda was dismantled to strengthen the base and back portion. The original stones were numbered and laid out over an area the size of a Football pitch, all properly numbered and waiting to be put back together (very much like a jigsaw puzzle). The pagoda was in serious danger of collapsing under its huge weight. Mureuksaji is the largest pagoda in Korea. The Japanese in 1915 built concrete supports at the back of the pagoda, helping the pagoda last another hundred years. When the pagoda was re-assembled this concrete portion was removed, making the pagoda whole again for the first time in about 400 years. The pagoda dates back to 639 AD, therefore it is one of the oldest stone pagodas in Korea as well.

The Wanggung-ri palace site is almost completely gone with only another stone pagoda (5-stories high), and some small stone pillars, foundations, and wall structures remaining. Much of the site has been occasionally closed for archeological studies. In the Iksan area alone, more than 19,000 Baekje artifacts have been unearthed. The sites in Iksan are interesting, but most visitors will come away somewhat disappointed if you don't also visit Gongju or Buyeo. I would recommend visiting some of the excellent museums to add value to the sites.
Baekje mostly demonstrates its OUV because of its regional connections and how it came into contact frequently with Japan and China along its coastal areas though trade and regional dominance. Upon inscription, Korea's Three Kingdom Period have now been recognized (Silla, Goguryeo, and Baekje) on the World Heritage List.
Flickr Album