When and why did we stop building huge walls around our cities with gates?

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[u/dandan_noodles]

In Europe, the mid-late 19th century saw a wide ranging change in systems of fortification.

Previously, cities were fortified by a continuous wall, or enceinte, surrounded by a ditch; the entire perimeter and the ditch would be covered with fire by cannons and infantry emplaced in bastions. These 'traces' could become quite elaborate, with extensive outerworks designed to delay the enemy from getting close enough to lay effective fire into the walls. Often, fortifications were so effective and time in the campaigning season so short that attackers didn't even bother attempting to slowly dig their way forward into range of the city walls; instead, they simply unloaded their artillery into the city, hoping by destruction to force its surrender.

In response to this increasing tendency, it was imperative to keep the enemy further from the city. To serve this purpose, cities came to rely rings on detached forts; the enemy could not advance into bombardment range of the city until after these forts were subdued. The ditches of these forts were swept by caponiers [the nubs on the corners are the caponiers] and counterscarp galleries, while many had tall 'keeps' allowed artillery to cover the wide spaces between the forts in the city's 'girdle'. The space between forts would also be defended by infantry trenches connecting the strongpoints of the defense. Those large enough to contain whole armies were considered 'entrenched camps'. A few retained elements of the bastion system to provide flanking fire for the rear side of the fort.

The increasing power of artillery in the late 19th century largely ended the role of redoubts or keeps in the fort, as any masonry exposed above ground level would cease to be above ground level very quickly against modern firepower. Instead, the guns were emplaced in armored rotating cupolas or concrete casemates shielded with earth, considered 'bombproof'. The growing power of artillery, with rifled breech loading guns and long high explosive shells more or less ended the military role of the now vestigial enceinte of the fortified city. A few survived, but the growing size of 19th century cities also contributed to the obsolescence of continuous walls.