The game board is different for each level in terms of its width, height, and terrain. We represent it by a 2D tile array. The grid, while not explicitly shown to the player, is fairly apparent from our tile drawing methods. Thus, the player is locked to working on a tiled board, and not a floating point area.
The player cannot build walls and designers cannot place rocks on the outer ring of tiles on the board. This prevents a bug in which the player can form a garden simply by connection sides of the board. However, this has also created a situation where bugs are able to funnel around the edge of the screen to different parts of the map. If this is desired, levels are built with this in mind. To prevent this, levels are designed with water on the edges, which cannot be crossed by bugs.
Note again that there are no rocks or walls on the edge tiles, but marshes are allowed.