A.27.1- The ability is written as "
A.27.2- A creature with landwalk is unblockable as long as the defending player controls at least one land of the specified type. [CompRules 2001/07/23 - 502.6b]
A.27.3- Landwalk of a particular type is a non-cumulative ability. Having the same type of Landwalk more than once gives no additional effect. See Rule A.1.2. [CompRules 2001/07/23 - 502.6e]
A.27.4- Landwalk is a static ability. See Rule A.3. [CompRules 2001/07/23 - 501.1]
A.27.Ruling.1- If the defending player does not control any lands of the appropriate land type, then the ability does nothing. [D'Angelo 1998/02/03]
A.27.Ruling.2- Even creatures with the same Landwalk ability cannot block such creatures if the defending player controls the appropriate land. [CompRules 2001/07/23 - 502.6d]
A.27.Ruling.3- The land type (if it is a basic type) can be affected by Magical Hack or Mind Bend. [D'Angelo 1998/02/03]
A.27.Ruling.4- Landwalk is an evasion ability (see Rule A.15), so it is cumulative with other evasion abilities. [CompRules 2001/07/23 - 502.6b]
A.27.Ruling.5- Spells and abilities which look for a kind of Landwalk only find the exact kind of Landwalk specified. For example, a spell which targets a creature with Islandwalk will not work on a creature with Snow-Covered Islandwalk, as that is a different kind of Islandwalk. [D'Angelo 2001/09/17]
Note- Also see Snow-Covered Lands, Rule K.21.