Here's what rhymes with castellum.
                        
                        
                    
                                vellum  velum  cerebellum  flagellum  antebellum  Words that almost rhyme with castellum
harem  melon  felon  heron  antiserum  watermelon  denim  bedlam  seldom  welcome  venom  addendum  plenum  jetsam  phenom  spectrum  rectum  septum  unwelcome  webcam  plectrum  bubblegum  momentum  referendum  interregnum