A limekiln is a furnace used to burn limestone or shells at high temperature, reducing them to quicklime for use in mortar, plaster, or agricultural improvement. That such a structure was tucked inside the standing corner of a 13th-century castle says something quietly telling about how post-medieval inhabitants related to the older fabric of the town: not as monuments to be preserved, but as convenient raw material and ready-made enclosures.

The excavations, carried out at the Custom House site in Galway's Townparks townland, uncovered a remarkable concentration of medieval remains in a relatively confined area. The eastern part of the site, bordering Druid Lane, contained a high medieval hall and a smelting works; the western part, inside the warehouse, yielded the castle corner and the kiln. The limekiln itself survived as the southern half of a circular, bowl-shaped structure set into the eastern corner of the castle. It measured 3.5 metres across at the top and narrowed to 3 metres at a depth of 1.25 metres, which was as far as the excavation reached. Its interior was faced with roughly coursed red sandstone and green granite boulders, and a flue nearly 2.75 metres long ran southward from the base of the bowl, passing across what remained of the old castle wall. Once the kiln fell out of use, it became a rubbish pit. Its deposits contained oyster shells, animal and fish bone, occasional bird bone, and a substantial assemblage of medieval pottery including Redcliffe ware, produced in Bristol, and Saintonge ware from south-west France, both of which circulated widely in Irish port towns during the high medieval period. The ash and lime deposits confirm what the structure's shape already suggested, but it is the discarded shellfish and imported ceramics that give the site its texture, a small, grimy corner of a functioning town going about its business long after the castle that once framed it had crumbled.