Open the inventory window by clicking on a Storage House. Then, click once on the following items. A red triangle will appear in the item's square as each is selected.

Row 1, item 3 (wooden boards)
Row 2, item 2 (iron bars)
Row 2, item 4 (wine casks)
Row 3, item 1 (bread)
Row 3, item 5 (cooked meat)
Row 4, item 1 (animal hide)
Row 4, item 5 (plate mail)
Row 5, item 1 (axes)
Row 5, item 2 (swords)
Row 5, item 3 (lances)
Row 5, item 4 (pikes)
Row 5, item 5 (long bows)

Then, click on Row 6, item 1 (crossbow) to add 10 of each item to your inventory or on Row 6, item 2 (horses) to complete the current level.