William Earl Marshall built fortifications at Thomastown, fragments of which can still be seen today, together with nearby Grennan Castle, now in ruins. Of this castle and the town's walls, the only remains are the towers near each end of the bridge and the remains of a 13th-century church, dedicated to St Mary. The town became a small medieval walled town.
The cache is a micro camouflaged container placed near the railway bridge on Station road Thomastown.