Masterson is key man
The reigning champions opened impressively, leading 0-4 to 0-0 after just 11 minutes through Tomás Sheehy, George Sunderland, Conor Carty and Andy Merringan.
Points from Philly Cullen and Paul Murphy free hauled Sarsfields back into the game and Castletown were dealt a real setback when Kevin Morris was sent off.