When the Russians controlled Finland they built a nationwide network of fortifications. Afraid of a Barbarossa style attack and capture of the then Capitol St. Petersburg. Helsinki was also fortified although most of these fortifications were never used. Now much is destroyed or ignored, while some houses have old fortifications running through their gardens.

Links

Krepost Sveaborg – A great site to find out about the Helsinki Fortifications in particular.

HS has taken some photos (list) of how they are now – zoom out on the map to about 10km and you will see what I mean.

Finland for Thought there are some good comments here too.

hugovk has some good photos.