In theory, North Korea has a number of natural features that could be used as Bastions. The west coast is rugged with numerous offshore islands forming numerous straits, channels and inlets between them and the coast, perfect for Romeo-class submarines to hide among (yes, there are three on the west). The east coast has a flatter coastline with very few natural obstacles, except for Wonsan Bay where the Tapchon Joint Naval Base is being built nearby - a relatively deep and enclosed bay with a number of small islands and peninsulas that act as natural barriers. North Korea could rely on those natural terrains, along with its growing arsenal of shore-based anti-aircraft and anti-ship missiles, as well as its formidable and battle-tested coastal mine warfare and submarine capabilities, to protect them.
However, the "Bastion" concept would probably be more appropriate for the modified Romeos, as their technical characteristics imply that they would probably stick to protected waters - the propulsion system remains the same while its rudders is quite small for the size and weight of the ship, a clear implication that the ship has poor speed and underwater maneuverability and is only capable of basic diving and maneuvering. Kim's choice of nuclear propulsion for his new submarines - with all its advantages in range and speed - was clearly not to keep them close to shore in closely guarded waters. The construction of large surface combatants with good seaworthiness only further proves that.
It was clear that Kim wanted his navy to extend farther offshore, specifically to compete for dominance in the Sea of Japan. But given Germany's past lessons, is it too risky and adventurous to follow that path that has proven to be a costly failure? Or is Kim building his navy on a bet on deep cooperation with Russia and China, which could provide military support (and hopefully not be abandoned or sold out by them)?