Suomenlinna is nice place to visit.I have recorded a video from suomenlinna in winter.  It’s really beautiful with the snow and everything.I really recommend visiting Suomenlinna Fort if you are visiting Helsinki. I used my go pro camera to record footage so sorry for not perfect quality.

https://www.youtube.com/watch?v=U2ZZOWFqfsw&feature=youtu.be – Suomenlinna Fort