A roundhouse normally doesn't have any special scoring involved; it is simply a nickname for a set of other things that score, that happen to add up to 24. In your case:
Double Royal Marriage: 8
Queens around: 6
Kings around: 8
3 Marriages: 6
This adds up to 28 (which is 24 for the roundhouse, plus 4 for the extra royal marriage).
Thus, by default rules, you should have scored 28.
There are variants that some people play where a roundhouse does score higher than 24. However, you and your playgroup would have to discuss and agree to such a thing before the game; as it isn't part of the standard rules.
This consists of a king and a queen of each suit. It normally scores 24: royal marriage + 3 marriages + kings around + queens around. However some players give a roundhouse a higher score of 32. Some score a roundhouse with a run as 39 (24 + 15, allowing the royal marriage to contribute to the roundhouse as well as being part of the run). Some score a double roundhouse as 240 (it should normally be 160 = 8 + 3*4 + 80 + 60).