This page contains links to other webpages that discuss Period Cube. There's also a few reviews in there just because some of them are quite interesting. To be honest, there's not really that much on the Web in English about the game other than reviews, but hopefully the stuff I've linked here is good enough. Please note that some pages may contain spoilers.
Official Website (In Japanese)
Hinano's rather hilarious review of the game
r/otomegames Period Cube Discussion Thread
Every single blog post I've ever written about Period Cube
A really interesting review of Poyo-Poyo's route (The comments section is really good too!)
Reviews of some of the drama CDs in English
Character popularity poll results (In Japanese) (To view the results, scroll down and click the white button.)
A Link to the PS Store where you can download Period Cube Vita Themes (Please note, I have no idea if they're free or not, and they're only available if you have a Japanese PSN account.)
If you yourself have any interesting Period Cube related links that probably belong here, don't hesitate to send them to the email firstname.lastname@example.org!
Other Otome Related Sites
Kiss or Gossip
Miki's Otome Oasis
Hinano's Game Babbles