kingdom does have a sort of "underbridge" and its walls don't have much of an impact (compared to goldrush and its accursed oak log pillars)
iirc it doesn't have gravel tho
The map design though. Literal dogwater. Wool Mainly in the nexus. Not realistic, flat dirt not only bad looking but not creative.
The walls may not have an impact but the map design is just bad.
In thebridge, there will always be a consequence for every action. If you are mining, you need some time before reaching nexus to defend. Same with defending, if you are defending you cannot farm gold/iron to gear up. Which is why all the mines in the original maps are far away from the nexus. This makes it so that the rusher has a better chance to break the nexus, at least 3 or 4 times. It's balanced.
However, in kingdom, ALL the mines are in the center, and nexus in the dead center. This makes it so clumped and very hard to deal with, which is why these maps tend to have long games.
This also forces you to limit your meta, only wanting to rush when you have gear. Causing every single game on kingdom being a pot rush map. It makes it repetitive and not fun. Invisible is practically impossible with no place to run or hide after breaking nexus. Pressure plates can deal with them very easily considering the flat ground.
And for the 1 block off to void nexus, it's very dangerous and defeats mole mining in thebridge (break and run!) Strategy. Very easy for swappers to camp hence lengthening the games even longer.