The King Charles Hotel would be a very good hotel for almost every reason except the utterly terrible noise environment.
The staff are friendly and attentive when available, although that is only a short period in the morning during the cleaning/breakfast period. Otherwise it's self-check in which worked very well and a mobile number for any issues. Not a problem for us on this trip but if there were a need for reception support it isn't readily available.
The breakfast is quite good with many options both hot and cold. To rooms are well presented and generally it is a visually nice hotel.
However, and this is a highly significant issue, the hotel is on the busiest and loudest street in Valletta and situated exactly above the very loudest bar on that street. Every afternoon there is a happy hour that begins with loud music and continues on until the early hours. The bass reverberates through the building and you can hear every word of every song. We had to use earplugs and noise cancelling headphones to be able to get to sleep at any point before the early hours, and that wasn't always possible. This is not somewhere you can return to and relax after a long day sightseeing and frankly makes any stay unpleasant.
It is a real shame because this is a decent hotel right in the heart of Valletta, but unless you're going to be one of the merrymakers enjoying the party until the early hours it's one to avoid like the plague.
Under no circumstances would I stay here again.