Complex sentence: Unsurprisingly, due to the near-yearly sieges the city had suffered, it "had reached such a state that naught but its name remained"; it is perhaps because of this that so little of pre- 12th century Polonnaruwa remains today.
Split into simpler sentences: Unsurprisingly, due to the near-yearly sieges, the city had suffered and had reached a state that nothing but its name remained. It is perhaps because of this that so little of pre- 12th century Polonnaruwa remains today.