What's 250km (160mi) long, 4km (2.5mi) wide, and ironically named? Why, the Korean Demilitarized Zone, or DMZ for short, of course! It is considered to be the most heavily armed border in the world. With nearly 1.2 million active duty military personnel, North Korea has the 4th largest standing army in the world, and when paramilitary and reservists are included, the largest military in the world. This is in part an outcome of the Songun, or "military first" policy in North Korea.
An interesting tidbit I learned some years ago is that North Korea tries to maintain roughly a 2:1 ratio in active military personnel when compared to South Korea. Given that South Korea currently has 630,000 active duty military personnel, it would seem North Korea is managing to roughly maintain this ratio. Furthermore, I believe the placement of the military members is also maintained to some degree at this ratio. There should be roughly twice as many North Korean soldiers guarding the border as South Koreans.
A North Korean soldier stands guard in Kaesong.