Carmack worked on the engine. Romero built the dev tools in addition to designing levels. Both contributed significantly to the design of the game. Carmack is

Carmack worked on the engine. Romero built the dev tools in addition to designing levels. Both contributed significantly to the design of the game. Carmack is one of the most talented engineers alive, but that doesn't negate all of the work that the rest of iD put into Doom.