There are two Roman watch-towers at Shielhill in Tayside, located to either side of the main military highway into north-east Scotland. The southernmost tower (Shielhill-South) lies to the west of the road while the northern tower (Shielhill-North) lies to the east. Both of these fortifications consist of a four-post watch-tower enclosed by an earthen rampart surrounded by two external ditches and an entrance facing the road. The two watchtowers lie about 1,000 yards (c.914 m) apart, Shielhill-South some 1,000 yards north of the similar watch-tower at Blackhill Wood, Ardoch, while Shielhill-North lies 1,000 yards to the south of the fortlet at Kaims Castle.
Shielhill North: This sub-rectangular watch-tower measures about (c.24 m) in diameter with its double-ditch system interrupted on the south-east facing the road. When examined in the early 1970's, the inner ditch was found to be 5½ ft. wide and nearly 2 ft. deep with a fill of turf, ash and burnt clay, the outer ditch 4½ ft. wide and 1½ ft. deep containing a filling of natural peat. A single round post-hole uncovered at the time had been packed with gravel, the top 12 ins. of which were removed from the hole when the 1 ft. square timber post was removed, the hole then being filled with gravel, earth, charcoal and pieces of daub. This shows that the tower, like its neighbour to the south, was dismantled after a short period of use, probably during a general withdrawal from north-east Scotland.
Shielhill South: This watch-tower was also defended by two ditches, the inner was 50 ft. in diameter, the outer 77 ft. in diameter with an entrance-causeway facing the Roman road on the SE. The four post-holes of the timber tower 11½ ft. square, had all been filled with cobbles and capped with clay after the original timbers had been rocked-out, this and other similar "filling-in" on other sites along the Gask Ridge is eloquent evidence for an ordered withdrawal of the Roman military from the area.
The regular spacing of these small fortifications along this particular stretch of the military road into the north-east, and others along the Gask Ridge constitute the first planned military frontier in Britain, the precursor of both Hadrian's Wall in northern England and the short-lived Antonine Wall in southern Scotland.