Wales was conquered; the Crusades had effectively closed even if they didn’t want to admit it. The Hundred Years’ War between England and France continued to rage all through the 14th century, off and on, taking turns with the plague for destructiveness. But during this century, central governments became stronger, in general. Europe’s castles had always been primarily outposts of strength that balanced regional against national power. (cf. Gilbert de Clare’s Caerphilly Castle, begun during a time of disloyalty to his monarch). Increasingly, that’s what they stood for. Their builders wanted strongholds to live in, so that they could survive in a time of uprising or civil war.
Most of the castle’s engineering was identical to the real working castles built in the 13th century. However, a few things changed.
Residential comfort mattered more. The 14th century was the era of tapestry and tiled floors. The Spanish princess who married England’s Edward I had brought along thick tapestry-like fabrics that her servants spread on the floor of the castle room appointed for her. The English were amazed that anyone would walk on something that took so much work to make, but it wasn’t long before Northern Europe’s wealthiest nobles were ordering carpets. The weavers of Flanders began cranking out wall tapestries by the hundreds, although it took about a year to make one. Dedicated bath-rooms had wooden tubs lined with linen, tiled floors, heat right in the room, and sometimes piped-in running water.
At the same time, the feudal system was breaking down. Lords were more likely to have paid men at arms, and sometimes they were foreign mercenaries. After a period in the 13th century in which the well-guarded Keep was less a priority, masons began to design defensive Keeps again. Why? In case the mercenaries turned against the family and its most loyal retainers. Nobody had ever considered that a castle might need to be held from inside, but the new luxurious keeps were built that way.
You can’t have everything in a castle without Kanye West’s budget, so they skimped on some of the grimmer defensive structures, like murder-holes. It mattered more to design latrines that actually carried the sewage smell away from the residential windows. So they still designed the castle around the site’s defensive features (cliffs, rivers), and they still dug moats, but walls were slightly less massive. Effort went into fine stonecarving instead of a second portcullis. Shipping stone blocks from a quarry was sometimes too costly, given the likelihood of a real siege. In such cases, castles were built of local stone or even brick. Brick actually had an advantage: if set on fire, brick walls were less damaged than stone ones, since they had already been baked in a kiln.
All castle windows required glass by now; nobody wanted simple wooden shutters to block out winter. Rooms intended for grand use, like the lord’s bedchamber, the hall, and the chapel, needed Gothic arches and tracery to support smaller panes of clear glass. This is one clear marker for later-built castles. Another is that rooms are divided into smaller units with more partitions. Expectations were changing; more people expected semi-private bedrooms. It’s possible that knights no longer accepted bunking down on the hall floor.