Regardless of whether the Castle Suite is a standard suite or not, it did appear on the website as eligible for SUA and the Hyatt agent was able to book it for me. If it was an error I would've appreciated if the hotel had given me notice before arrival date that the Castle suite should not have been made available for SUA, because there was a Plaza suite available for upgrade at the time, or I could be given the option of canceling the upgrade request.

At check in I was told the Castle suite was already booked and that they would've given it to me if it was available so it's not the case that they considered the Castle Suite to be a premium suite. By that time all the other rooms were booked, so I ended up wasting a SUA for an upgrade that I didn't want.