The place is always called by the Swedish name of Kastelholm and, sadly, hte website is Swedish only. I couldn't find one worth looking at in English about it.
It's a FREE open-air museum near to the castle and easily reached by bus from Mariehamn. The adjacent prison museum is no longer free so perhaps the life of this as a free museum is limited. Anyway it's good.
none needed -see below Kastelholma Ahvenanmaa Finland