He was one of the first organists to play fugues by giving the subject first, and have the other voices follow in succession. He also extended the use of the organ pedals and was one of the first to use the pedals for the voice in a fugue. He was also a very good teacher as many of his keyboard works were written for his students. His influence was widespread, as his music was known in England. He had earned the nickname 'The Orpheus Of Amsterdam' and the city fathers would bring guests from the surrounding area to hear him play.
He evidently spent his entire life in Amsterdam, but his expertise on organs was in high demand so he traveled inspecting and testing organs and giving advice on their construction. After the Calvinist Reformation of the church in Amsterdam, organ music was no longer allowed during church service. Sweelinck would give impromptu recitals on the organ an hour before and an hour after church services. These impromptu recitals were very popular, as Sweelinck would play the popular tunes of the day and then improvise variations on the tune.
Sweelinck left about 70 compositions for keyboard, and a glimpse of his powers as an improviser can be heard in these pieces. The Variations on Est-ce Mars is one such example. The tune is French and was well-known at the time. The first line of the song roughly translates to: "Is that Mars, the great god of battles, that I see?" The words may not mean much to modern ears, but Sweelinck shows his imagination and skill in the seven variations on the tune.
As with all music that is so old, there are performance practices of the time that we know little about, if anything at all. Without knowing how these pieces were actually performed, especially in a time where improvisation was much more prevalent, any modern performance may be but an approximation. Be that as it may, the music of Sweelinck and other composers of so long ago can still be listened to and appreciated, especially if a sensitive musician is playing the music. Music can be a powerful form of expression and can bridge the centuries.