Townhouses are considered private dwellings that are attached to similar structures on?

Townhouses are defined as private dwelling units that are usually attached to similar structures, which can be one or more on neighboring sides. This characteristic allows for varying configurations; some townhouses may share a wall with another unit on one side only, while others may have units attached on both sides. Therefore, the option that specifies "one or both sides" accurately reflects the flexibility in the design and layout of townhouses. This versatility is what distinguishes townhouses from other types of residential structures, such as single-family homes, which are detached and do not share walls with adjacent units. The correct answer encapsulates the various possibilities of townhouse attachment configurations.