Complex sentence: Instead, the Spartans held Thermopylae for three days and, although they were ultimately annihilated, they inflicted serious damage upon the Persian army, and most importantly delayed its progress to Athens, providing sufficient time for the city's evacuation to the island of Salamis.
Split into simpler sentences: Instead, the Spartans held Thermopylae for three days and, although they were ultimately annihilated, they inflicted serious damage upon the Persian army. Most importantly, this delayed the Persians'progress to Athens, providing sufficient time for the city's evacuation to the island of Salamis.