Castells can be divided in three ways: by height, by the base structure (with one, two or three bases, which stand one on top of the other) and by the number of people on each stage.
Height. Castells can be 6, 7, 8, 9 or even 10 stages high. All castells have a pinya (the base that takes the weight of the tower structure - the broader the better), the tronc (the structure of castellers who climb up on top of each other and which can be from two to five stages high; it also sets the degree of difficulty of each castell) and the pom de dalt, the top three stages consisting of the dosos, the aixecador and the enxaneta, made by the youngest and smallest members of the group. Castells with 6 stages are an essential part of the repertoire, 7 stage towers are regarded as being a basic skill, while 8 stages are ranked as advanced. Castells with 9 stages (mostly with folre) are within the capabilities of very few groups. Those with 10 stages are rarely seen (having been successfully dismantled only twice in the last 200 years).
Castellers on each stage. Castells can be raised with two, three, four or five people per stage, though exceptionally there can be seven, nine or even twelve. When a tower has only one person per stage they are called pilars. The pilars are build at the beginning and at the end of a performance.
Each tower is named according to the height and the number of castellers per stage. For example, an eight stages tower with three people per stage is called tres de vuit (three by eight).
There are also special towers, because they are complex estructures (for example, castells with a pilar in the middle) or because they are build with a different tecnique (as raised from below castells).
The bases. The pinya is the base of the castell, but some larger constructions have what is known as the folre, a second smaller pinya, on top of the first. Some of the taller castells even have a third pinya, known as the manilles. The same kind of castell (for example, one of 9 stages with 4 people on each stage) can be raised with or without the folre.