For the biggest percentage of Blind Guardian's songs, particularly in more recent years, he has almost strictly played rhythm guitar, with most lead and solo work being performed by André Olbrich.

Growing up in a Roman Catholic family, Siepen claims to not be religious, and that he doesn't believe in God.

"[1] Since 1995, Siepen has been an endorser of Mesa/Boogie amplifiers, using various Triple Rectifier heads and multiple preamps.

As of 2010, his main rack consisted of a Triple Rectifier "Recto Reborn" head, with a Fractal Audio Axe-FX II providing effects.

Up until the late '90s, Siepen mostly used ESP guitars with Blind Guardian co-guitarist André Olbrich, both guitarists making use of custom-made Horizon and Mirage guitars, although Siepen has been seen using other shapes, such as a custom-built model based on the Jackson Randy Rhoads.