Complex sentence: It was one of the more powerful among the Etruscan 12 ‑ city confederation, and like Cortona just up the Valdichiana, the lowlands around Chiusi are a treasure trove of tombs for this civilization.
Split into simpler sentences: It was one of the more powerful among the Etruscan 12 ‑ city confederation. The lowlands around Chiusi are a treasure trove of tombs for this civilization.