Joseon fortresses?
Joseon wasn't waring state like Qing or Japan during Sengoku period, there really wasn't need for many fortresses as country was largely peaceful, and especially after the Neo-Confucius scholars domination military development were strictly forbidden hence during Joseon times there was more scientific & scholastic development than building fortresses and developing weapons.
However, many of early Joseon fortresses are concentrated at the Hamgyeong province (far North-eastern parts of Korean peninsula) where frequent Jurchens and other nomadic incursions, and Kyongsang province at South-eastern parts of Korean peninsula for defending against to frequent Wokou (wajin or Wa, Japanese pirates) incursions.
The early days of Joseon fortresses were direct copies of late Koryo (i.e. Goryo) Dynasty, early forms of Goryo fortresses were famous for using the mountainous terrain as the base for the fortress then build the walls around with slabs of stones. later Goryo fortresses started using river or ponds to make fortress for protective from invading infantry and seize machines. Which was the standard of many other fortresses in East Asia. Nothing was special about Goryo fortress other than it's mostly mountainous in design.
Early Joseon fortress copied this same basic designs:
1. 장성(長城) - Jangseong - basic long walled castle/fortress (made mostly from stones) made for defense or blocking the wide area.
2. 관문성(關門城) - Kwanmunseong - design to blockage the entry or paths.
3. 산성(山城) - Sanseong - mountain fortress.
4. 전성(塼城) - Jeunseong - walled fortress or castle build from bricks. (Very few in Korea, mostly in China)
The typical early Joseon fortresses were Sanseong (mountain fortress) because mountains are accounts for over 70% of Korean peninsula, and using the nature as defense was cheap and very effective, also Korean mountains connects to other mountains in Korea hence it also provided effective communication mechanism.
Mostly long walled castles or fortresses were from Gogureyo or Baekje & Silla times, because during three-kingdom periods in Korea is known for warning states period where frequent wars required Jangseong.
However, with introduction of many gunpowder weaponry such as cannons, Joseon started deploying Jeunseong types, and one of last surviving fortress is Hwaseong (
http://en.wikipedia....aseong_Fortress)
More pictures and detailed information at
http://www.hwasong.h...savenije.pe.kr/Here is the various mountain fortresses:


And this is the Koryo mountain fortress

And this is one of entrance for Hwaseong (brick style or later Joseon fortress style)