My advice is simply to put "tower" and not translate "prismática" at all.

The point is this, I think: in English, by default, the word "tower" means what is strictly, in geometric terms, a prismatic tower. It is extremely rare for architectural descriptions in English actually to call towers "prismatic" or "prism-shaped", since it's taken for granted. But in Spanish it's really quite common to refer to "torres prismáticas" in Romanesque architecture, and apparently it is also common in Czech, since most of the instances of "prismatic tower" that you find by googling it are from Czech sources.

All "prismática" means is that the tower is a right prism, in geometric terms. That is, it has a polygonal base, most often square or rectangular, but sometimes hexagonal or octagonal, and that the sides are vertical, so it has the same section all the way up. This is so notwithstanding Phil's example, which is actually a modified prism that widens at the base.

https://en.wikipedia.org/wiki/Prism_(geometry)

Well, when we use the word "tower" in English that is what we mean. The only Romanesque towers that are not "prismatic" are round towers, of which there are some, or "espadañas", which are called bell-gables in English and are not classed as towers.

"Siguiendo al profesor García Guinea, el románico montañés presenta tres tipos de torres: la torre prismática (Castañeda y Cervatos) la torre cilíndrica (Santillana y Elines) y la espadaña."
http://www.romanicoenruta.com/cantabria/cantabriaruta07/stam...

English regards "prismática" as redundant. Omitting it will lose nothing, in practice, since a tower is prismatic unless otherwise stated (if it's a round tower, and the shape matters, you call it a round tower). Including it will simply mark your translation as a translation: no English writer would have said it.