In order to play chain similar tiles together by clicking, dragging, and releasing to clear them.
Swords and Shields give you temporary 3 turn buffs indicated by the circles next to your current Attack and Defense in the upper left corner. In order to deal damage enemies you must link them to weapons or each other. Damage is dealt to HP by subtracting ATK from DEF.
Gold can be collected to purchase items from the store. The store is only accessible when no monsters are on the board, and items can be purchased by dragging items from the shop directly into your inventory in the appropriate slot.
Crystals can be collected to build up the ability to teleport which clears the currently play board along with all monsters. Be careful though, some enemies are persistent!