LEAVE IT! IT'S NOT WORTH IT!

Why are there no town walls around Mumbles, but there is still a big F-off Castle? 

Tenby has walls, Conwy has walls, even Laughne has walls, yet Mumbles has none. Apparently, its because it was a pokey little hole not worth defending! 

From Gemini:-

There are no town walls around Mumbles because it was historically a small, rural fishing and farming village rather than a fortified medieval borough.

Historical Status:- Village, Not a Borough

During the Middle Ages, nearby Swansea grew into a planned, prosperous trading town that successfully petitioned for and built defensive town walls. Mumbles (historically centered around the village of Oystermouth) never received a charter or status as an urban trading borough that warranted extensive civic fortifications. 

The Role of the Castle: Instead of a walled town, the community relied on Oystermouth Castle for defense. Built in the 12th century, the castle sat on an elevated limestone ridge to project military power and protect the local lords of Gower, rather than enclosing the civilian cluster of houses below with a continuous perimeter wall.

Geographic Isolation: For centuries, Mumbles was physically separated from Swansea by marshland and sand burrows. It did not develop into a populated, connected residential and commercial hub until the 19th-century growth of the Oystermouth Railway and the booming local oyster industry, long after the era of town wall construction had passed.

So there! Now you know.