I didn't say it has a more complex map. I said a more complex level design, meaning that the levels themselves look more complicated in architecture detail, geometry, etc. Ultima is like endless tunnels and corridors with very little geometry detail and even has a short draw distance. Dooms levels look complex with many details and geometry while having unlimited draw distance.
The above image of Ultima was the most complex/detailed looking i could find. While Doom has even more complex looking architecture in later levels.
In all fairness Ultima Underworld was a little more advance than Wolfenstein 3D in some respects. As it has textured walls and floors, and varying degrees in height for floors and walls that could be at 45 degree angles. Three things that the Wolf3D engine couldn't do.
The most advance version of the Wolf3D engine was used for Rise of the Triad, which did add textured floors and ceilings and diminishing lighting (fog) as well as a few other tricks. But it was still stuck with a 90 degree wall limitation and could not do elevated floors or ceilings. RotT faked higher and lower areas on the map by using sprites that the player could stand on.
The Doom engine is much more advanced though, as it has walls that could be displayed at any angle, has sector based lighting for shadow and highlight effects, used binary space partitioning, which allowed for large areas to be rendered at fast speeds as it would render only what is on screen in relation to the player. And it also has skyboxes.
The Duke 3D Build engine was simaler to the Doom engine, but was a little more advance as it could do sloped floors and ceilings, voxel rendering support (Shadow Warrior and Blood displayed voxel sprites in areas), and also used teleportation tricks to fake rooms over rooms and pools of water that the player could swim in. It could also be used to create DR. Who like TARDIS rooms. Build could also do moving sectors that could be used for things like trains or the spinning objects.
Ken Silverman's Build engine was a pretty brilliant take on the Doom engine for its day. Even John Carmack was impressed by it, but also described it as 'an engine that felt like it was being held together by bubble gum and paper clips', or something like that. But a few months after Duke 3D was released, Quake came out, and the Quake engine was a whole new level of tech. But I do think that some of the features from Build were also achieved in later source code ports of the Doom engine.
