The origin of the word burgus is disputed by researchers: either a Germanic (CIL XIII 6509) or else a Greek origin (πύργος; pýrgos; Jos. BI 1,99-100) are assumed. The word, which appears before the middle of the 2nd cent. AD and is still attested under Valentinian I, denotes a small fortified watchtower (ILS 396) or is used as a diminutive for castellum (Veg. Mil. 4.10).; the burgus was generally used for surveillance (CIL VIII 2494-2495: burgus speculatorius).
1 D. Baatz, Bauten, MAVORS XI, 1994, 83-85.