Though the castle bears the English monarch's name, its construction actually spans several phases, with the earliest portions possibly dating to around 1200. The site itself is even older; excavations in 2016 uncovered the remains of a souterrain in the western courtyard, suggesting the castle was built atop an earlier promontory fort. The fortress is constructed from locally sourced limestone blocks and greywacke, with its distinctive D-shaped design reflecting the military architecture of the early medieval period.

The castle's layout reveals its complex building history through its two distinct sections divided by a massive cross-wall. The western portion, with its polygonal courtyard and rectangular gatehouse, represents the earliest phase of construction. This section features numerous defensive elements including narrow arrow slits set within large embrasures, a square projecting tower at the southwest corner that becomes five-sided at first-floor level, and evidence of barrel vaulting in the northern gate tower. The eastern hall, likely added around 1262 according to payment records for stone quarrying and transport, showcases later medieval defensive improvements. The thick dividing wall between these sections, measuring about 3.3 metres at its base, clearly blocks earlier openings in both the north and south walls, confirming it as a later addition to strengthen the castle's defences.

Archaeological evidence points to significant modifications during the late medieval period, particularly in the southeast corner where punch-dressed limestone arcading and multiple fireplaces suggest a sixteenth-century renovation that transformed this military stronghold into a more comfortable residence. The basement level contains a barrel-vaulted passage with its original wicker centring still visible, whilst upper levels show evidence of wooden floors supported by corbels and the massive cross-wall. A spiral staircase in the northeast corner provided access between levels via mural passages, and post-medieval additions include what appears to be a gun platform on the eastern wall, demonstrating the castle's continued military importance well into the early modern period.