Why is the "s" in fortsätter pronounces as /ʃ/? Is it something to do with the "rt" being next to the s?
Yes and no, but mostly yes. They're not separated, since in a cluster of R + D/T/N/S, the entire cluster is made retroflex by the R.
Ok thanks! Retroflex is still something I have to wrap my head around as it doesn't occur in English but I'll get it. :)