Tower of Death

Just finished this novel.  Have to say I couldn't figure how the floor could drop and nobody notice.  Very elaborate gimmick, to say the least.  The clews were all there although the optical illusion was kept in reserve.  Not fair, Mr. Gibson.  All in all a very enjoyable read.  I was very upset with myself for having read "The Death Clew" before this one.  I've been trying to read the stories in the order they were written and I made a mistake and flipped these two novels.  The last paragraph of "Tower of Death" references "The Death Clew".  I don't know if that had been done before or since but it was the first time I can remember a passage in the story leading the reader to the next issue.  That would have hooked me in the day.