On Start the Week Kirsty Wark explores what it means to live either side of a wall, and whether barriers are built to repel or protect. Supporters of the US President urge him to 'build a great wall' along the Mexican border but the journalist Ed Vulliamy points out that there is already a wall and border guards, supported and funded by US Presidents for decades. And yet still drugs, guns, money and people continue to move north and south. Israel has been building its own separation barrier since the turn of the century, but Dorit Rabinyan is more interested in psychological barriers that drive Palestinians and Israelis apart. The map-maker Garrett Carr travels Ireland's border to explore the smugglers, kings, peacemakers and terrorists who've criss-crossed this frontier, and asks what it will become when the United Kingdom leaves the EU. The historian Tom Holland looks back at the successes and failures of wall-building from Offa's Dyke to Hadrian's Wall and asks whether they work more as statements of power than as insurmountable barriers between people.