Shell small (<3 mm), somewhat high, strong, robust, bicarinate and umbilicate.
The protoconch measures 260-290 µm and has a little more than 1,5 whorls. There are two distinct phases, the first, approximately 3/4 of whorl, is completely smooth and the second, which is partially concealed by the teleoconch, presents randomly distributed thick tubercles.
The teleoconch has 2 whorls and two thick carinae, one peripheral and another basal, that angle the shell markedly.
Ornamentation formed by cords and spiral grooves, ribs and axial folds. The adapical part is fully covered by spiral cords of different sizes, not nodulose, developing in zigzag, with 3 cords at the beginning of the teleoconch which become 16 at the end, between the suture and the peripheral carina; some of these cords are divided into two finer ones near the lip; the space between cords, on the first whorl is occupied by oval cells that are transformed into grooves on the last whorl; next to the lip, 3-4 thick axial ribs develop irregularly in shape and very close together, thickening the outer lip and projecting the peripheral carina outward. The periphery is delimited by two strong carinae which angle the shell; between both carinae, fine spiral cordlets are developed, as well as ribs and thick undulating folds, numbering 9-10 spiral cordlets, 10 ribs and 12 thick axial folds on the last whorl.
The base is slightly convex and is covered by spiral cords which are divided into two as they approach the lip. Umbilicus narrow and deep, not limited by any carina. Aperture quadrangular, prosocline, inner and outer lips very coarse, expanded outwards; parietal area covered by a thick callous coating forming a marked angle; columella arched, very reflected towards the umbilicus and developing an extensive callous area at the base; outer lip modified by the two carinae that angle it internally and expand it outwards.
Dimensions: the holotype is 2.67 mm in diameter.

No other studied species has the morphological characters observed in A. fastigata. The species is characterized by its robustness; the
strength of the carinae; the small size of its protoconch; the thick axial folds that develop between the carinae; the axial folds that develop on the adapical part,
next to the lip, that project the peripheral carina outwardly; the narrow and deep umbilicus; the thick quadrangular aperture and the lips projected outwards.

Only known from Papua Nueva Guinea, at 10 m.