When the wicker was removed or rotted away, it left faint impressions on the vault surface, and those traces survive here still, readable by anyone who knows what they are looking at. It is not a dramatic ruin open to the sky, but a largely intact subterranean structure, cleared of modern debris and incorporated into the amenity area of the offices above.

Archaeologist Brian Hodkinson investigated the undercroft in 1989 as part of a broader programme of excavation on King's Island, work prompted by landscaping requirements between the castle and the new Civic Office Complex. His findings revealed two distinct building phases. The first produced a substantial outer shell measuring 22 metres east to west and 7 metres wide, set into the slope of the ground to the east, with a doorway in the middle of the north wall and a possible second entrance through the west wall that may have given access directly to the river. Windows were present in all walls except the west. The second, later phase saw the addition of the arches and barrel vaults that define the space today. A five-arched central spine wall divided the interior into two long bays, with corresponding arches in the side walls aligned on the original Phase 1 openings. In the northern bay, the vaulting changes direction around the entrance area, an adjustment understood as a practical solution to provide sufficient headroom at the doorway. Pottery sherds recovered from trenches against the outer wall pointed to a 13th or 14th-century date for the earlier phase, while the vaulted additions are considered late medieval on the basis of the wicker-centring evidence.

The site sits to the south of King John's Castle on King's Island, in the older part of the city known as Englishtown. Because it has been incorporated into the civic complex rather than fenced off as a conventional heritage site, it can feel easy to overlook. The medieval city wall traced during the same excavation ran south from the castle's south-west tower along the riverside, so the undercroft sits within a corridor of significant below-ground archaeology. Visitors should look for the wicker impressions on the vault surfaces, faint concave marks left by the curvature of the formwork rods, and pay attention to the shift in vault direction in the northern bay, where the builders solved a practical problem with a quietly elegant piece of structural thinking.