It should not be understated though how good the toolset was for the time. The original idea for Keen came after they (Carmack and Tom Hall) recreated the first

It should not be understated though how good the toolset was for the time. The original idea for Keen came after they (Carmack and Tom Hall) recreated the first level of Super Mario Bro's 3 in one night. They had to break from Softdisk under Romero's suggestion, because Softdisk would claim proprietorship over the engine. They actually "borrowed" their work computers from Softdisk over a weekend to work on Keen and created the first level in 72 hours. After the success of the first Keen it was a no-brainer to keep making more and milking it for all they could.