The term 'building up' can be summarized by the crossword solution 'amassing'. Amassing is the act of collecting or gathering a large number of items, often for a specific purpose. It is a process that involves accumulating and organizing resources in order to create something larger or more complex. This could include collecting money, materials, or other resources in order to construct a building, or it could involve collecting information or data in order to build up knowledge or understanding.