A mystery blaze threatened to destroy Abergele's Gwrych Castle on Tuesday.
Fire fighters from Abergele, Rhyl, and Colwyn bay rushed to the unhabited castle late on Tuesday afternoon after smoke was seen coming from the smashed windows.
Fire crews wearing breathing apparatus went into the castle and prevented the blaze spreading beyond the ground floor, but not before part on the floor collapsed into the cellars, leaving a hole 12ft square.
Station officer Terry Edwards from Rhyl , leading the fire teams, said the cause on the blaze had not yet been established, but a fire investigation unit would be searching the building on Tuesday evening.
"When the first crew arrived there were clouds of smoke coming from the roof and most of the windows on the west side, and flames could be seen through the windows," he said.