Island Continuity

Certain squares must be part of an island to avoid creating a wall area of 2x2. This can result in other squares also needing to be part of the same island. Once an island is completed, it can be surrounded by walls.

Here's an example:

The square marked with a red dot must be part of an island to avoid a wall area of 2x2. This means the square to its left must also be part of the same island. The island of 3 is now completed and we can surround it with a wall.