When Tideturner is hidden, can it be played into the base to switch with another friendly unit on a battlefield, or must it be played at the battlefield where it was hidden?

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Tideturner must be played to the battlefield where it was hidden. You cannot play it to your Base while it is Hidden. After it is played to that battlefield, its play effect may choose a friendly unit at another battlefield and swap locations with it, because the card's play-effect targeting cannot be satisfied at its hidden battlefield and so may be chosen from other locations [811.1.d.1] [811.1.d.2].

Tideturner must be played from Hidden to the battlefield where it was hidden, not to the Base [811.1.d.1] [355.2]. However, when its

You hide Tideturner only at a battlefield you control (Hidden places a facedown card at a battlefield you control) [811.1.b]. When you play Tideturner from Hidden, step 355.2 requires you to choose the location where the unit will be placed; because it's played from Hidden, that location must be the battlefield where it was hidden [355.2] [811.1.d.1]. As the unit is played, its 'When you play me' play effect can choose a friendly unit as a target. The rules explicitly treat Tideturner as an example whose play-effect target may be chosen from other locations because the ability's restriction makes choosing a target at its battlefield impossible, so you may pick a friendly unit at another battlefield and then move/swap them [811.1.d.2]. If you don't want the Hidden-play restriction, you may instead play Tideturner normally from your hand (not from Hidden), in which case you can play it to your Base or any other valid location at normal timing [811.3].

If the printed text of Tideturner lacked an explicit 'at another location' clause and instead simply said 'choose a friendly unit,' the rules still provide the example treating Tideturner as having an effect that requires targeting a unit at another location; if ambiguity remained in a practical judge situation, the most defensible interpretation is to follow the official example in the rules text [811.1.d.2]. If no legal target exists at resolution (e.g., you chose a unit that becomes illegal), instructions that can't be followed are ignored per the targeting/ resolution rules [359.3.e.6]. Also note that Hidden cards gain Reaction while facedown and can be played as Reactions in allowed windows, but that doesn't change the requirement that a Hidden permanent be played to its hidden battlefield [811.6] [811.1.d.1].

Tideturner's Hidden status imposes a location restriction on where it is played from Hidden: a hidden permanent must be played to the battlefield where it was hidden [811.1.d.1]. For Units that are being played, you must choose a valid Location where the Unit will be placed as part of the play process [355.2], and playing from Hidden makes that choice the battlefield where it was hidden. Therefore you cannot choose your Base as the play location while playing Tideturner from Hidden; the Hidden play forces it onto that battlefield [811.1.d.1] [355.2].

However, the rules expressly cover Tideturner as an example in the Hidden rules exception: if a hidden permanent's play effect chooses targets but that targeting restriction can never be satisfied at the battlefield where it was hidden, then the play-effect's targets may be chosen from among available options elsewhere [811.1.d.2]. The rules use Tideturner as the example of this case, explaining that because its play ability requires choosing a unit at another location, that target may be chosen freely. Practically, that means you play Tideturner from Hidden to the battlefield where it was hidden, then its 'When you play me' play effect can choose a friendly unit at another battlefield and the two units swap locations [811.1.d.2].

If you prefer to play Tideturner to the Base (or to any location without Hidden restrictions), you must not play it from Hidden. The rules allow a card with Hidden to instead be played normally (not from Hidden), and that normal play has no Hidden-imposed targeting/location restrictions [811.3].

In short: playing from Hidden = must be placed on the battlefield where it was hidden, then its effect can move it to another battlefield by swapping with a chosen friendly unit (per the Tideturner example) [811.1.d.1] [811.1.d.2]. Playing it to the Base is only possible if you play it normally (not from Hidden) [811.3].