I mean. the theme is very similar. dark and gothic but thats not really half of it. the biggest thing is the fact that each enemy felt dangerous(for the most part) and combat was very much tied to whatever weapon you had.
but the biggest similarity is the boss fights. I mean, castlevania is the melee dodge and then attack boss fight kind of game. you won't win anything buy running at bosses and hitting them mindlessly. you have to play defensively. in dark souls its mostly a defensive game where you make your attack after a successful defensive movie most of the time.
outside of the platforming which is missing from souls games. the whole enemy layout and boss fights is how a castlevania plays. enemies scattered around areas in the game with no context obviously its just expected to have these creatures in such a world. and boss fights are pretty much a non scripted room where you have to recognize patterns of mostly melee attacks .