This is an interesting development on North Korea's northern border:
Since late 2013 North Korea has been fortifying some parts of its [northern] border. Some units have managed to scrounge up concrete and rebar (reinforcing steel rods) and built machine-gun positions. Otherwise troops have dug trenches and reinforced them with lumber. At the same time efforts to prevent North Koreans from leaving have been increased. Security on the border (including cell phone detection teams) has sharply increased. The state controlled media is talking about the possibility of a Chinese attack and the presence of Chinese spies disguised as other foreigners.
I'm assuming from context that the post means the northern border.
In the Cold War, I always read that more Romanian military assets were pointed north to watch the Russians than were poised to attack NATO. So there is precedent.
Of course, North Korea is actually making it easier for China to invade by facilitating bridge and road building projects.
If North Korea has unrest--which the North Korean rulers are increasingly worried about--China might very well march south to put a more reliable and friendly government in place; or, in the worst case of a collapse China can't prevent under new rulers, to keep the South Koreans (and America) as far from the Yalu River as possible.
It could be a dangerous race. I hope the North Koreans do more than build speed bumps up there.