Valletta is compact, but it is not flat. The main bus terminal is accessible, ferries can remove some of the steepest walking, and route
133 helps along the ridge and down toward the harbor side.
If mobility matters, build the day around one clear spine from
City Gate through
Republic Street, then add only one or two nearby anchors such as
St. John's Co-Cathedral or
Sacra Infermeria & Mediterranian Conference Center. That approach is usually calmer than trying to collect every staircase and bastion edge.