a base part;

a top part;

two mutually spaced side parts extending from the base part to the top part, and inclined cross parts each extending from one side part to the other and having an upper end and a lower end, the inclined cross parts being arranged one above another and inclined in opposite senses, the inclined cross parts being rigid and being connected to the side parts;

wherein the lower one of the two successive oppositely inclined cross parts has its upper end attached to an articulation node to which the lower end of the upper one of said two successive oppositely inclined cross parts is attached; and the articulation node further being attached to one of the side parts, a section of the side part opposite said articulation node, which section extends between the upper end of said upper inclined cross part and the lower end of said lower inclined cross part, being collapsible so that said upper inclined cross part together with the portion of the tower above it can be lowered into a position in which said upper inclined cross part extends substantially parallel to said lower inclined cross part, a releasable locking device being provided to prevent collapse of the collapsible section,

said section comprising an upper portion and a lower portion which are pivotally attached together, and the locking device comprising a lever which is mounted on one said portion and which is movable between a locking position, in which it is engageable with the other said portion to prevent collapse of said section, and a release position.