If you've come to Dingestow Castle expecting dramatic ruins you are 840 years too late: the castle was destroyed in 1183! According to the 1857 Antiquarian Excursions in the Neighbourhood of Monmouth, the castle was being built by Ranulph de Poer, Sheriff of Herefordshire, when it was "attacked at break of day by the young men of Gwent, with the descendants of those who had been treacherously slain at Abergavenny by William de Braose" (see The Ogre of Abergavenny GC76JB1). It was rebuilt subsequently, but excavations in 1969 failed to reveal any stonework at all.
Luckily some impressive banks and ditches remain: a Motte and Bailey described on Coflein as "two enclosures, the upper being sub-rectangular, about 54m by 38m, the lower adjoins on the south-east and is about 36m by 60m." It is worth walking up to the north-east corner of the motte (castle mound) just above the GZ and looking north along the valley of the River Trothy. From there you can appreciate how Dingestow Castle and its twin Motte and Bailey in Mill Wood, 400m to the east, would have commanded the surrounding area, where the low-lying, north-south orientated Trothy Valley joins the larger low-lying area between Raglan and Monmouth in an otherwise hilly landscape.
Park just to the NE of Dingestow Church (taking in GC4TG8R) and walk NW along the lane to enter the Castle field at the wooden gate, or walk another 100m along the road and there's an alternative small entrance closer to GZ. Please close all gates because sheep often graze the Castle Mound. The cache should be an easy find, though tucked out of site of people wandering around the castle site (with permission from the landowner and grazier). There is space for TBs.