St. Petersburg is a memorable cruise destination, and Catherine's Palace is just one of its many amazing sights. 
While in St. Petersburg on the Silversea Silver Cloud, we toured Catherine's Palace with a delightful local guide named Alla Ushakova. Her perspective of the history and culture of St. Petersburg was both informative and mesmerizing.
This tour of the palace shows just how opulent it was during the days of the Tsars.
Amber Room in Catherine's Palace (c) Linda Garrison
St Petersburg is a wonderful cruise destination, and even better visiting it on a luxury cruise ship like Silversea. I love to visit St Isaacs Cathedral as well as the Hermitage – you can spend days in that place! Like you say, Catherine’s Palace is a must see place too.