Castle Greg is a Roman fortlet on Camilty Hill in the middle of a fly-infested bog on the foothills of the Pentlands near Camilty, South-east of West Calder on the North side of the A71 "Lang Whang".

Castle Greg is not named after a warrior called Gregory! According to Wilkinson, the greg element is Celtic craig, not least since the name was given as Castle Craig in the C19th (West Lothian Place Names, p.18). However, an earlier form of the name is Castelgreg (1512) and as any visitor to the site will soon observe, there are no craggy formations nearby, which leaves a question hanging over the derivation of the C19th form of the name. Wilkinson therefore suggests a derivation from Welsh grug, meaning "heather", which certainly fits the boggy character of the location more accurately!

One further linguistic factor which Wilkinson draws attention to is the possibility that the fort was known to the Romans as Camulosessa Præsidium, connected with the nearby Camilty, which, as Cameltree (C18th) may derive from Brythonic Camulos Tref, meaning "village of Camulos". Camulos was, of course, an ancient Celtic pagan war-god often compared with the Roman Mars, whose name is remembered in other Romano-British placenames such as Camulodunum (appearing in Colchester and Almondbury, both in England), and is celebrated in a Latin inscription found in the Roman fort on Bar Hill, Dumbartonshire: "Deo Marti Camulo" (Anne Ross, Pagan Celtic Britain, p.234).

To add a further confusing connection, the Caiy Stane, a few miles to the East, has also been known as the Camus Stane, which may suggest another dedication of some kind to Camulos.

However, according to Watson, Camilty derives not from a Brythonic root but from Gaelic cammalltaidh, which he defines as "crooked little burn" (Celtic Place-Names of Scotland, p.143). Certainly, this area of the Pentlands also contains other Gaelic placenames, such as the nearby towns of Balerno and Currie (baile airneach and currach respectively).

According to Collard, Castle Greg is "one of the best preserved Roman earthworks in the country" (Lothian: A Historical Guide, p.47). First excavated in the C19th by Daniel Wilson, the remains of the fort consist of a clearly visible (although un-photogenic!) rectangular rampart of some 38x50m:

An entrance is visible on the East wall, now marked by a slight depression:

A system of double ditches can be found around the Western, Southern and Eastern walls:

The visitor to Castle Greg will notice the perhaps surprisingly remote location of the fort, which also has practically no long-distance views available to it. One possible explanation of this is that it formed a staging post along a main East-West artery, between the kingdoms of Gododdin (Lothian) and Strathclyde. Certainly, today it lies just to the North of the "Lang Whang", or A70, which performs just such a function.