An portcullis is gate behind or in front of an entrance. It is made out of wood and iron. They were made to keep out the enemies and they could be raised when there was no danger. A portcullis protects the entrance of a castle. It was pulled up by a rope. The portcullis was invented by the Romans in 208 B.C. and was on castles in the 12th century.
Picture from
http://www.castlexplorer.co.uk/glossary.php#p