and guess the title mentions "HarbourS" because some fortifications are not part of Valletta but of other cities (Birgu, Senglea, Floriana, ...). Also, the part north of Valletta is called Marsamxett harbour while the southern part is called Grand harbour.
Exactly the conclusion I suggested - those looking on a map from afar might regard all the inlets as being "one harbour" around the capital "Valletta". In fact they are separate ones with separate names - hence use of the plural for "harbours" which are close together and hence, despite the title "Harbours of Malta", which could be regarded as implying a wider distribution around the island, there is no need (as Ralf was wondering) to look beyond the immediate "Valletta area" for locations of this TWHS.
We have now identified 9 locations for this TWHS on this Web site - up from 1 yesterday. As you have visited "most (all?) of the knights fortifications
" would you like to suggest any further additions for the benefit of others?