The Ggantija temple is on Goze, the second island of Malta, and is the largest megalithic temple on Malta. As with the other temples on Malta, the Hagar Qim, Mnajdra and Tarxien, the question is whether this ‘temple’ really was a place of worship or whether there was some other purpose. The fact that it is on such a small island is strange in itself: such a huge construction suggests that there was a large population, not only to help build it but also for its function as temple.
The architecture is typical for all temples on Malta: a shape that resembles a clover-leaf with boundary walls made of megalithic slabs at a few metres distance. This space between the two walls is filled with sand and stone rubble. Together this makes a solid wall which remains standing after many millennia.
The length to width ratio is roughly 40 to 30 metres and some of the megalithic slabs measure more than 6 metres in length. It is unclear whether it was ever roofed. The walls seem unsuited to the bearing of heavy megalithic roof plates.

In 1827 this temple was excavated. Since the excavation, done in a rather crude manner, the temple has suffered from the effects of vandalism as wel as harsh weather. 
The building of the temple is placed in the timeframe 3600 – 3000 BC but one should put a big question mark next to this since the foundation for the use of these dates has never been recorded scientifically.
Why is the biggest temple built on this tiny island? Or was it not an island at all in those days; perhaps it was part of the mainland? And who quarried and transported the enormous slabs?
It doesn’t make sense. Tiny island, no people around, a huge fortress-like building, why? Who were the ancient engineers?  

More about Ggantija in the book "Verborgen geheimen van de mensheid", and more in ref. Malta and Mayrhofer.