Decades after the release of Dark Castle for the MSX2, the game has finally been preserved online, thanks to the efforts of the game preservationist @TrevgauntletNeu.
Dark Castle was a dungeon crawler released for the MSX2 in Japan in 1993 and is one of three games that were developed by the indie developer MNS Soft (alongside Kidoairaku and Bunazogu). It was published using Brother Industries' Takeru kiosk system — a software distribution machine that would write video games to a cartridge or disk and was typically found inside Japanese video game stores in the '80s and '90s.
As @TrevgauntletNeu notes, in the past, very little information about the game was available online, except for a series of short magazine listings that appeared in several Takeru Press magazines, and only featured a single sentence.
This led some (such as The Lost Media Wiki) to suggest it was a lost MSX port of the Silicon Beach Software's game of the same name from 1986, but we now know that isn't the case.
According to @TrevgauntletNeu, the retro enthusiast located the obscure game inside the Japanese hobby shop Suruga-ya while on a visit to Japan, and couldn't believe their luck at finding the "real deal". Since then, they've gone through the process of making the title publicly available on archive.org, so you can now experience the obscure piece of video game history for yourself.
You can download the game here. It requires an MSX2 or 2+ with at least 128kb of RAM.