There can be no doubt that Castlevania was one of the killer apps for the NES. While it is basically just a side-scrolling platform action/adventure game of which there were so very many on 8-bit and 16-bit platforms, it was one of the earlier ones, released in 1987. It invented, enhanced or popularized many of the play mechanics and standards that would be used in such games in the future.
Castlevania was also a departure from the typical side-scrollers of the time, at least on the NES, in that the theme was horror. Most similar games had a much lighter theme (Super Mario Bros. being the obvious example). In Castlevania, you played the role of Simon Belmont who has journeyed to Castlevania to defeat the vampire Count Dracula. Castlevania consists of 18 levels and your primary weapon is a whip. Like most games of this type, you can pick up various items and other weapons along the way.
Castlevania actually started life in 1986 as a game for the Family Computer (Famicom) Disk System which never made its way to the U.S. For U.S. release in 1987, it was ported to a standard cartridge but a cartridge release wouldn’t happen in Japan until 1993. Since that time it has been re-released as part of various compilations. In 2002 it was released for Windows as part of Konami Collector’s Series: Castlevania & Contra. In 2004, it made its way to the Game Boy Advance as part of the Classic NES Series. More recently, Castlevania was included as one of the 30 games on the NES Classic Edition released in 2016. It was also included in the Castlevania Anniversary Collection which was released for Konami’s 50th Anniversary.
Castlevania is definitely a game that any NES or side-scrolling action game fan should play. It isn’t terribly hard to find in original cartridge format but you can also get the NES Classic Edition or pick of the Castlevania Anniversary Collection which is available for Windows via Steam with the added advantage of including other classic Castlevania games. I prefer the original NES version but emulated is okay too. If you enjoy this game you will probably enjoy many of the other games in the series, including Castlevania II (NES), Castlevania III (NES), Super Castlevania IV (SNES) as well as released for the Genesis and DS platforms.