Both include mysteries, action (not as much as Nijuu, but enough) and adventure.
Just like how the littlest details are important to figuring out the mystery in Nijuu Mensou, the same goes with Gosick.
Both are set the 1900s (Nijuu Mensou-1950s/Gosick-1924) and have.
The main reason why I recommended Gosick is because they're alike in several ways and keeps you on your toes when you're watching the Anime.
(Just like in the Pandora Hearts Manga) Literally every detail contributes to working out the mystery.
Also, Gosick is cute.
Who doesn't think a 5' 0", almost doll-like, girl with very long blonde hair and emerald eyes with a tsundere personality isn't cute?!
These are both anime that follow a girl whose life changes as they are taken away to a life of danger and adventure (although they do adapt to this new life differently). These anime both had to do with solving problems and mysterious.
This anime is similar to nijuu mensou no musume because they both contain adventures however in ashita no nadja the girl tries to find her mother and no weapons are in much but in nijuu mensou no musume the girl leaves her house as she might die
Both stories unfold with professional thieves, and hunted all over the police network. Both have a certain mystery of "who is the thief? What his true face". There is also a somewhat dramatic mood compared to the girls in the series.
Both shows are about young girls on an adventure through an alternate timeline with elements of steampunk/diesel-punk and retro futurism. Daughter of Twenty Faces is a complete story though, and not nearly as episodic in nature as Princess Principal. If you're looking for a somewhat dark narrative with a strong female lead, you can't go wrong with Daughter of Twenty Faces.
This anime is similar to nijuu mensou no musume because they both have girls dealing and learning how to use weapons. In akatsuki no Yona the girl left her castle because her father was killed by her crush who later became the king and earns the 4 dragons as companions while in nijuu mensou no musume the girl leaves home to avoid being poisoned.
A mix of historical themes with modern issues. The Daughter of 20 Faces is set solely in the recent past, while Galilei Donna is more sci-fi fantasy, seemingly set in the present or near future, with many references to the distant past. Both are good stories mainly about risky adventure, and searching for a series of important items, all the while in constant dangers of being shot at by being everyone's enemies.
Not only both anime deal with some form of supernatural cases, they also share the same overall "mood" and even artstyle. Ryouko's Case File is set in the modern times while Nijuu Mensou is set in the fifties. If you like the pulp detective genre, you'll definitely find Ryouko's Case File and Nijuu Mesou no Musume refreshing. Although I must say that Ryouko's cases are reminiscent of X-Files...
Baccano is set in the earlier 20th century while Nijuu Mensou is around the mid-20th century. Both involves gangs, thieves, criminals and clans. Although the plot differs, they are very similar in terms of setting at least.