This is the 1650th most frequent Japanese word.
立体
Three-dimensional object or shape.
In this sentence, "立体" refers to a three-dimensional geometric figure.
この三角形の立体を描くのは難しいです。
Drawing a three-dimensional figure of this triangle is difficult.
Here, "立体" is used to describe the three-dimensional aspect of the building.
立体としての建物は美しいデザインです。
The building as a three-dimensional structure has a beautiful design.
In this usage, "立体" pertains to the concept of spatial (three-dimensional) thinking or reasoning.