When Charles I of Spain invited the Knights of St John to take possession of Malta, Birgu was the place they chose to settle, fortifying an area that swelled to become the Three Cities of Cospicua, Senglea and Vittoriosa, renamed in celebration of the Great Siege of 1565. Traces of the Knights are still visible throughout the Three Cities, though less so in Senglea, which was largely flattened during dive-bombing runs on the nearby dockyards during WWII. For tourist purposes, the Three Cities became the Four Cities in the late-19th century when the British got into the fort building act too, throwing up Fort Rinella to house the state-of-the-art, world's biggest cannon, Armstrong 100-ton gun (it was fired forty times and didn't hit a thing).
Buses from Valletta all stop directly outside Vittoriosa's Three Gates, a mammoth series of defensive structures which withstood the Great Siege and now, appropriately enough, house the Malta at War Museum (though the steep admission fee proves a deterrent against entry every bit as strong as anything the Knights ever built). It's a short walk downhill from the main gate to the Inquisitor's Palace, home of the Inquisition from 1574 to 1798 and breeding ground for two popes and twenty-two cardinals. The area behind here is the Collachio, starter home for the Order before its climb up the island's property ladder. Now a spruced-up warren of ancient streets, these days you're more likely to see t-shirts and ladies' underwear than muskets or swords poking through the upper windows. It's a good place to lose yourself in for a half hour or so, narrow streets ending in stretches of the original city walls, raised up to overlook Kalkara Creek.
The Collachio ends at Victory Square, on the other side of which streets tumble sharply to St Lawrence's Church and the marina, pleasure boats straining gently at their moorings on the crystal-dimpled water. Pass the Malta Maritime Museum, the Casino di Venezia and the super-yachts of the rich and you reach the huge, many-cornered slab of stone that is St Elmo's Fort, now closed to visitors but retaining the uninterrupted sweep of Floriana, Valletta and Senglea from the marina down below.
If you're walking to Senglea then Cospicua is on your way. Follow the road above the marina past the parish church, hang a left at the square with the golden statue and you'll see St Helen's Gate straight ahead, which gives you a half-decent view of what's left of the town. The outer gate, Poiverista, now doubles as a house and garage space and isn't worth the extra walk.
And then Senglea, where the best thing to see is Vittoriosa, back across the channel dotted on this side with innumerable small fishing boats, washed on the dock by the locals after they've finished sponging down their cars. You could easily stand and take a hundred different pictures - I know, I tried - and still not exhaust the views, though the best of all is from L'Isla Point, at the very tip of the peninsula, where a garden cut into the battlements looks out across the Grand Harbour to Valletta and the sea.
You'll need at least an afternoon to see the Three Cities on foot, or a full day if you're planning to see any of the museums. If you're really pushed for time then Vittoriosa - only a twenty-minute bus ride from Valletta - is the one place you really shouldn't miss.