Enemies appear at the entrance to the stage, usually at the top left corner of the screen, and move along the path to the exit. You lose one LIFE when an enemy reaches the exit and leaves the screen. You start with 20 LIFE, and more can be bought with Crystals, in sets of five LIFE.
Several enemies make up a wave, and several waves make up a stage. Enemies drop G and come in two types - LND (land) and AIR (flying) - and can not be damaged by some attacks, depending on their type. Some waves end with a single enemy different to all the others in the wave. This enemy is always LND type and moves reasonably slowly, but it has much more HP then the other enemies in the wave. These enemies drop Crystals at their exact location when defeated. Three of them end the final wave in each stage. When a wave is cleared additional G is obtained based on the current wave clear bonus and collected/accumilated G.
Crystals can be traded to the merchant at the top right of the map for additional LIFE, a higher wave clear bonus and new types of tower.
The player's job is to stop the enemies by buying Block Towers using G and hitting them with the Sword.
Block Defence has 7 kinds of tower. All kinds of tower have four levels. They are built by selecting them from a menu opened by pressing square in open land (not the path, not occupied by a tower and not where you can't stand). They do not snap to a grid, but should be placed carefully, as placements in a grid fashion allow the most towers to be built. The game always starts with the first 3 available, while 1 Crystal each buys the next 3 kinds of tower and 2 Crystals buys the last one. Buying a tower with a Crystal allows G to be spent to build as many towers of that kind as the player can afford and has room for. The towers are (Class/Classes it hits are in the brackets):
- Arrow (LND/AIR)
- Anti-Air (AIR)
- Cannon (LND)
- Fire (LND/AIR)
- Quake (LND)
- Slow (LND/AIR)
- Missile (LND/AIR)