Complex sentence: After a final assault on 1 June by troops, the top of the fort was occupied on 2 June and fighting went on underground until the garrison ran out of water and surrendered on 7 June.
Split into simpler sentences: After a final assault on 1 June, by troops, the top of the fort was occupied on 2 June. Fighting went on underground until the garrison ran out of water and surrendered on 7 June.