Portal was Wizards of the Coast's first major attempt at a set that new players could come into the game and understand. Back before the major rules changes brought forth by 6th Edition, Magic rules were more complicated; today, they are simplified with the addition of the stack. The three Portal sets attempted to mitigate the complexity.
As of October 2005Portal, Portal Second Age, and Portal Three Kingdoms are legal in Vintage and Legacy tournaments.
, all cards
No new mechanics were introduced with Portal, but there were several "simplifications" made to the game through things left out of the set. Instants did not exist within the set (instead, every "one-shot" spell was a sorcery; however, some sorceries could be played at particular times uncommon to sorceries but normal for instants; most of these cards have since received errata making them Instants). All rules text was in boldface. All flavor text and reminder (italicized) text was not boldface. To separate rules text and flavor text, the cards utilized a line with a slight bulge on both top and bottom; this makes the line look like a very elongated diamond. Rather than using the words "block", "graveyard" and "library", Portal used the words "intercept", "discard pile" and "deck". Portal cards had no creature types.
Second Age also did not have Instants, though a number of Sorceries in the set have since received errata from Wizards of the Coast retroactively changing their card type to Instants. It also carried over the different style for rules text. Portal Second Age cards dispensed with the "Summon" wording for creature spells, and were the first to word them as "Creature -- type" that was adopted with 6th Edition. In Second Age, "block" remained as it was, replacing "intercept" from Portal. Similarly, "graveyard" re-replaced "discard pile", and "library" re-replaced "deck".
Portal Three Kingdoms is the only starter level set to introduce a new keyword ability into the game: "Horsemanship", an evasion ability that works like flying in that creatures with horsemanship can be blocked only by creatures with horsemanship. Creatures with horsemanship can block creatures without horsemanship as well, but not be blocked by creatures with flying, likewise Horsemanship creatures cannot block flying creatures. As such, these creatures are effectively 'unblockable' by almost all other creatures. There was controversy when the set became tournament legal, as some players thought that cards with the horsemanship ability should be errata to have flying instead. Wizards of the Coast initially considered making this change but decided against it, stating that the creatures with horsemanship are rare and not aggressive enough to make an impact on vintage formats. Horsemanship has become an obscure ability due to the limited production and geographical release of the set, and Wizards have stated that it will be unlikely that horsemanship will return on new cards in the future. With the release of Time Spiral, Wizards returned a vast number of older pre-8th Edition keyword abilities and mechanics; horsemanship was not among the mechanics that returned. The Magic the Gathering Online set Masters Edition III included a number of cards from the Portal Three Kingdoms set including ones with horsemanship.