The first building constructed by the Anglo-Normans in Athenry was the castle, built between 1237 and 1240 by Meiler de Bermingham. Athenry Castle is a ‘hall keep castle’ comprising two main components; a keep and an enclosing wall. The keep consists of a large, rectangular building which originally had a spacious first-floor hall open to the roof and a viewing gallery. The basement was accessed from the first floor, probably by means of a ladder (not through the later inserted ground-floor doorway). In keeping with other 13th-century hall keeps, it has a first-floor entrance, a projecting latrine/garderobe, crenellated battlements and a battered (outwardly projecting) base. It also contains fairly unusual cross-shaped arrow-loops in the crenellations. The gabled roof now present at the castle is a later addition. The keep and bawn (castle grounds which included ancillary buildings) are enclosed by a strong curtain wall with two towers, arrow-slits and a battered base. 

The most unusual and important features of Athenry Castle are the richly decorated first-floor doorway and windows. The foliage motifs and decorative devices were commonly employed in Romanesque architecture and continued to be used into the 13th century in the west of Ireland, in what has been termed ‘School of the West’ architecture. Capitals decorated in that style survive at Christchurch Cathedral in Dublin and date to around the turn of the 13th century, and can also be seen at Corcomroe Abbey, Co. Clare dating to the first quarter of the 13th century. That ‘Transitional’ work between Romanesque and Gothic architecture in the castle accords with the documentary evidence that suggests a construction date of c.1237–40. The style, mouldings and decorative motifs also occur in the Dominican priory at Athenry and support the castle’s early-to-mid 13th-century date. The presence of ‘School of the West’ architecture also reflects Gaelic influence, as it is most commonly found in ecclesiastical buildings built under the patronage of native Irish lords.

One other interesting aspect of Athenry Castle concerns how it was constructed. Until fairly recently it was considered that the keep underwent three construction phases and began life as a low, squat, single-storey building that was raised to two storeys soon after, and again, to its present height, in the 15th century. More recent research, however challenges this hypothesis, and also how the castle was restored. Based on research on the keep at Trim, Co. Meath, the practice of constructing roofs well below the top of the walls, which necessitated outlets for rainwater penetrating the thickness of the walls. These low-level roofs within great towers appear in the 12th and 13th centuries, not only in Ireland, but also in England (e.g. Tower of London), Wales, Germany and Switzerland. Other features indicative of low-level roofs are (i) blank walls without original openings around the upper levels of the tower, (ii) evidence for water outlets well below the tops of the walls, (iii) roof lines on the inner surfaces of the walls. Two of those features, (i) and (ii), clearly exist at Athenry; the entire upper half of the walls contains almost no openings, and drainage holes in the end walls at second-floor level indicate that rainwater originally ran off from a roof set at a low level within the tower. At a later date (probably in the 15th century) gables were built on the end-wall parapets supporting a new roof at that level (surviving today), however, no new windows were broken through the wall for the new second floor but there were small high-level windows incorporated into the new gables.

Structural evidence inside the hall (beam holes) also suggests that the keep did not have a second floor, but rather a timber gallery that ran along the south gable wall. The line of the gallery floor is visible in a pre-restoration photograph. The gallery would have been reached by a timber staircase, located at the north end of the gable wall, opposite the doorway (in a similar position to today). It is proposed that the gallery, while also fulfilling more recognised roles (e.g. accommodating musicians or minstrels), primarily functioned as a means of access from the first floor to the battlements, via an intramural stairway (i.e. built within the thickness of the wall).

Ultimately, although the keep looks tall and imposing from the outside, its interior originally comprised only a first floor hall open to the roof, a gallery and a ground-floor storage basement. It would therefore appear that at the time of building (1237–40), the outward expression of power and lordship was paramount and that the symbolic dominance of the landscape and population took precedence over the functional use of space.