Opulent Castle is a card where you can discard multiple cards for coins, but they must have certain characteristics (be victory cards). So that suggests that the cards would need to be individually revealed in the discard pile to make sure that all cards discarded are indeed victory cards. Dominion Online currently does not show the cards discarded beyond the top card. Obviously people can't cheat this way in an online game which enforces the rules, but in real play you would probably need to confirm that the cards discarded are indeed victory cards.
I think it should show all the discarded cards.
BTW is there a possibility to choose which card will be on top?