Both of them are from Jun Mochizuki, so there are a lot of similarities everywhere. Main character, Claudia, looks like a younger version of Alice (main character from PH) and not only this, chara design is generally very similar! This manga is like a little sister for Pandora Hearts... And has a lot of bishonens and very good plot too!
Crimson Shell was a sort of early incarnation of Jun Mochizuki's later work, Pandora Hearts. Both have her signature drawing style, and several of the character designs are similar. Both have similar settings and themes: An organization was instituted to fight against monsters, there is a girl who is one of those monsters but is different and more powerful. She has been forced to join the organization. A crucial part of the story is her trust in an older man, and how it seems that trust is betrayed. Lots of intrigue and magic. CS is only one volume, though.
Both are the first works of two mangakas that often have their most famous stories compared: Jun Mochizuki (Pandora Hearts and Crimson Shell) and Yana Toboso (Kuroshitsuji and Rust Blaster). Both stories give us the same feelings of their next and biggest works, you can see many similarities in the story, themes and design of the characters. Both stories (Crimson Shell and Rust Blaster) have also dark and kinda sad themes and endings.
Both Vassalord and Crimson Shell are gothic manga whit nice character design and similiar artstyle. Both are romantic and a bit gory, with much blood. Also, Xeno from Crimson Shell reminds me off Johnny.
Karneval and Crimson Shell have a story surrounding an organization that's trying to rid the world of something. This organization allows in new comers, which are some of the main characters to each story. Character wise, there are some similarities: Gareki (Karneval) and Xeno (CS) and Tsukumo (Karneval) and Claudia (CS) are the only ones I can really make - and the second one is only minor details. The feels for each story are pretty close as well.