Why are the walls of fortress Europe so high?

ECONOMIST: Visas in Europe

At worst, it might even step up its military presence in Transdniestria, to make a second Kaliningrad: a Russian fortress in south-east Europe.

ECONOMIST: Troubled Moldova

Orlando Figes, professor of history at London's Birkbeck College, says Mr Putin's home city of St Petersburg may be a window on Europe, but it can also been seen as a fortress against foreign opponents.

BBC: How Putin is inspired by history