build up

verb

• To erect; to construct.
• To close up by building.

  "to build up a door"

• To accumulate, to pile up, to increase in stages.

  "Ever since the secretary left, the letters in my inbox have started to build up."

• To strengthen.

  "They had to build up their fortress to protect against attack."

• In solitaire card games, to place a card over another card of lower value. (e.g., place 5♦ over 4♣)