Complex sentence: There, it claims that shelling began on 29 March, when 150 mortar shots and 60 cannon shots were fired, and during the next three days only the mortars fired 100 shots total, per day.
Split into simpler sentences: There, it claims that shelling began on 29 March, when 150 mortar shots and 60 cannon shots were fired. It continues, saying that during the next three days only the mortars were used, firing 100 shots per day.