Bath city walls the Romans, to surround their town (known then as "Aquae Sulis"), probably in the 3rd century. The Anglo-Saxons by the 10th century had established Bath as a fortified "burh" (borough), utilising the existing town walls, and maintaining Bath as a centre of regional power. Bath, located on the northern edge of the kingdom of Wessex, would have guarded against any attack from neighbouring Mercia, which was held by the Danes for a time. The height of the walls was increased on the orders of King Stephen during The Anarchy. Bath's medieval walls included four gates. The North and South Gates
Aphanizomenon flos-aquae toxic chemicals released when cells die. Once released (lysed), and ingested, these toxins can damage liver and nerve tissues in mammals. In areas where water quality is not closely monitored, the World Health Organization has assessed toxic algae as a health risk, citing the production of anatoxin-a, saxitoxins, and cylindrospermopsin. Dogs have been reported to have become ill or have fatal reactions after swimming in rivers and lakes containing toxic "A. flos-aquae". Microcystin toxin has been found in all 16 samples of "A. flos-aquae" products sold as food supplements in Germany and Switzerland, originating from Lake Klamath: 10 of 16