Historical Rules · 1 October 2012
Rule 301.6
Official source text from the 1 October 2012 Rules release
Some artifacts have the subtype "Fortification." A Fortification can be attached to a land. It can't legally be attached to an object that isn't a land. Fortification's analog to the equip keyword ability is the fortify keyword ability. Rules 301.5a-d apply to Fortifications in relation to lands just as they apply to Equipment in relation to creatures, with one clarification relating to rule 301.5c: a Fortification that's also a creature (not a land) can't fortify a land. (See rule 702.65, "Fortify.")