Hard to breach, easy to defend. Everything you need to survive hordes of the undead.
Shime Coal Mine - Kyushu, Japan
Source: commons.wikimedia.org
Tower Kielder Reservoir - Northumberland, England
Source: commons.wikimedia.org
Flakfortet Fort - Copenhagen, Denmark
Source: commons.wikimedia.org
Maunsell Forts - Kent, England
Source: commons.wikimedia.org
