Recent drilling has helped confirm that the deposit extends for 2.5km with "excellent grade continuity" along strike and at depth.
The latest assays included 86m at 8.2% total graphitic carbon from 104m, 53m at 11.5% TGC from 120m, and 9m at 11.4% from 3m, including 2m at 24%.
The drilling data follows recent metallurgical tests that produced a 95.1% TGC bulk fine graphite concentrate.
Castle is keen to get the project development ready to take advantage of an expected supply shortage later this decade, and has its sights set on the battery anode material market.
The company's 149sq.km licence contains 13 priority other targets it plans to test with auger drilling soon.
Castle is raising up to A$1 million via a share purchase plan at 1c, to top up its last reported cash position of $2 million.
In addition to its legacy position in Ghana, Castle also has assets in Western Australia, and has just started drilling for lithium at Woodcutters, close to the Bald Hill mine.
Castle shares were up 10% earlier today to 1.1c, capitalising the explorer at $14 million.