A mysterious field inhabited by strange life forms. Many evolved Bydo will attempt to lash out at you.
The Den is the fifth stage of R-Type, in between the Frontline Base and Transport System. The Den teems with Bydo lifeforms and pilots will have to deal with this bio-organic onslaught carefully.
In the Sega Master System version, the instructions refer to the level as "The Underwater Caves"
Bydo
Trivia
This setting also appears in both R-Type Command and R-Type Tactics II: Operation Bitter Chocolate (two times in Command but only once in Operation Bitter Chocolate).