Complex sentence: When the Last Alliance defeated Sauron in, Minas Ithil was restored as a city/fortress though it was now under the rule of Anárion's son Meneldil, as Isildur departed to take up rule of his father's kingdom in Arnor.
Split into simpler sentences: When the Last Alliance defeated Sauron in, Minas Ithil was restored as a city/fortress. Then Isildur departed to take up the rule of his father's kingdom in Arnor.