Level 9

From IFWiki

British text-adventure game company, founded by the Austin brothers (Pete, Mike and Nick) in 1981. Level 9 produced about twenty text adventures from 1981-1991, and thus was the major supplier of interactive fiction for British microcomputers. As was the case with Adventure International and Infocom, Level 9's early success was due in large part to its creation of a development system, called A-code that produced highly portable game files.