I have been unable to find any primary reference to Iscalis other than in Ptolemy’s Geographia: all later references derive from Ptolemy. The original was in Greek and I don’t know how the name was spelled as I can’t find a Greek text (of which the earliest manuscript is 13th c.)
The (2nd c.) Antonine Itinerary didn’t cover this area. William Stukeley’s 18th c. ‘reconstruction’ marked the Fosseway, and Ilchester as ‘Ischalis’ – the common identification at the time; but there is no Iter along the Fosseway in the original, so this is Stukeley’s addition. The nearest Iter is XV which goes through Winchester, Salisbury and Dorchester to end up at Exeter.
The Ravenna Cosmography also does not mention it so Ptolemy, c. 150 AD, appears to be the only early source.
The only other point of information (or more probably misinformation) was that it was one of the πόλεις of the Belgae, along with Venta Belgarum and Aque Calide. And, according to Ptolemy’s method, it was situated west of Aque Calide (Bath). One difficulty is that the territory of the Belgae almost certainly didn’t extend even as far as Bath, still less further west to ‘Iscalis’. Another suggestion (Francis Haverfield in the Somerset Victoria County History, 1906) is that it was no more than a mistake, perhaps a confusion with Isca Silurum, modern-day Caerleon.
If we discount the supposition that the town may not have existed at all, and if it did it wasn’t in the territory of the Belgae, we can say that it was in Somerset, probably in the area where the territories of Dobunni, Dumnonii and Durotriges met – the No Man’s Land. It also seems an irresistible conclusion that the name is related to the British isca, meaning ‘water’? (rather than ‘a river’ – afon – Abona). So we have a Romanised/Latin name with an adjectival suffix that suggests ‘a watery place’.
So, what manner of place was No Man’s Land at the time of the Romans?