Stopped building castles

Castles were great defences against the enemy.

However, when gunpowder was invented the castles stopped being an effective form of defence.

The use of gunpowder made both castles and city walls much more vulnerable because a cannon could knock down the stone walls.

The only way to knock down walls before the invention of gunpowder was to dig under the walls to make them unstable.

This was a highly dangerous way to do it especially as you might get boiling hot tar poured over you from above!