When your walls are level, you have a problem. Usually you want your walls to be plumb.

As a ~~pediatric~~ pedantic but fun point, in surveying, level lines are defined to follow the Earth's curvature. This means they are technically not straight. Horizonal lines, which are perpendicular to plumb lines, are often used as approximations for level lines.

Generally it's safe to use them interchangeably, but sometimes the distinction is important.

https://theconstructor.org/surveying/terms-in-leveling-uses/20077?amp=1

We do have examples of buildings needing to account for the curvature of the earth.

One that comes to mind is The fountain at the Christian Science Mother Church in Boston…

It’s about a quarter mile long and the water flows evenly over the granite curbs along its entire perimeter. If it didn’t follow the earth's curvature the water would all just spill over at the middle.

Im very surprised that noone here has mentioned the LHC in CERN Headquarters, Geneva, Switzerland. The LHC or Large Hadron Collider is so huge that it stretches across 2 countries (France and Switzerland) and stretches over a circumference of an impressive 17 miles (27 kilometers). It is so large that when building it they not only had to take into consideration the curvature of the LHC tunnel, but also the curvature of the earth.

You know how they say that gravity curves space-time?

And you remember from geometry class they told you all the rules you were learning were for “Euclidean” geometry, which had to do with flat space, and then they never mentioned it ever again?

Well basically this is a great example of where non-Euclidean geometry exists in the real world. Gravity bends space-time.

I guess you could put it on ever higher stilts to satisfy the parallel and level thing.

I seem to recall that this has to be accounted for when designing the support towers on particularly long bridges. According to a quick Google search, the most extreme example is the Akashi-Kaikyō bridge in Japan. The support towers are almost 3.5 inches farther apart at the top than they are at the bottom.

Also FYI, the word "level" generally refers to things laying perfectly flat. The word "plumb" refers to thing standing perfectly upright, which is why some jokers in the comments are giving you a hard time, despite what you meant being fairly obvious.

You can. The middle of the room just needs to be slightly recessed into the ground. Honestly, building sites are leveled before construction begins and they deal with a lot more variation than you’d see from the curvature of the earth.

FYI level refers to horizontal (mostly, although horizontal is a perfectly straight line 90° from vertical at that location while level follows the curvature of the earth), plumb for vertical.

I can do it. Two walls: one on the north pole and one at the south pole would be parallel and plumb.

Pretty sure they just level the ground though

Relatedly, some long structures that seek entirely straight have different lengths depending on the altitude you measure them. For example, an oft repeated tidbit is that the Lake Ponchartrain causeway is two inches longer when measured at the deck than at water level due to the earths curvature.