How does Bolas's Citadel work?
Bolas&#39;s Citadel doesn't allow you to cast anything for free.

To play a Planeswalker card or a Creature card is to cast it as a spell. You will need to pay life instead of mana if you play one from the top of you deck.



A spell is a card on the stack.[1] It doesn't matter if it's an instant, a sorcery, a creature, a planeswalker or something else. Cards of any type can be spells.[2] All that matters is that it's on the stack.

Bolas's Citadel imposes an alternative cost to spells when they are cast from the top of your library.[CR 118.9] Alternative costs replace the mana cost of spells.[CR 601.2f]

total cost to cast
= mana cost or alternative cost
+ additional costs and cost increases
- cost reductions

In this case, to cast a card from the top of your library, the cost would be calculated as follows:

total cost to cast
= mana cost or [life equal to the card's converted mana cost]
+ additional costs and cost increases
- cost reductions

For example, if you wanted to cast Llanowar Elves from the top of your library, you would have to pay 1 life (but no mana).

Spells cast from elsewhere (e.g. from your hand) are cast normally (because they aren't "spells cast this way").




It could also be a copy of a card on the stack (e.g. see Isochron Scepter) or a copy of spell (e.g. see Bonus Round).[CR 112.1, 112.1a, 112.1b] 
Lands are never cast, so they never go on the stack, so there can never be a land spell.[CR 305.9]

Bolas's Citadel does allow you to play a land from the top of your deck, but it's played normally (because it's not "a spell cast this way"). It even counts towards the limit on the number of lands per turn.[CR 305.2]





  112.1. A spell is a card on the stack. As the first step of being cast (see rule 601, “Casting Spells”), the card becomes a spell and is moved to the top of the stack from the zone it was in, which is usually its owner’s hand. (See rule 405, “Stack.”) A spell remains on the stack as a spell until it resolves (see rule 608, “Resolving Spells and Abilities”), is countered (see rule 701.5), or otherwise leaves the stack. For more information, see section 6, “Spells, Abilities, and Effects.”
  
  112.1a A copy of a spell is also a spell, even if it has no card associated with it. See rule 706.10.
  
  112.1b Some effects allow a player to cast a copy of a card; if the player does, that copy is a spell as well. See rule 706.12.
  
  118.9. Some spells have alternative costs. An alternative cost is a cost listed in a spell’s text, or applied to it from another effect, that its controller may pay rather than paying the spell’s mana cost. Alternative costs are usually phrased, “You may [action] rather than pay [this object’s] mana cost,” or “You may cast [this object] without paying its mana cost.” Note that some alternative costs are listed in keywords; see rule 702.
  
  305.2. A player can normally play one land during their turn; however, continuous effects may increase this number.
  
  305.9. If an object is both a land and another card type, it can be played only as a land. It can’t be cast as a spell.
  
  601.2f The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If multiple cost reductions apply, the player may apply them in any order. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be {0}. It can’t be reduced to less than {0}. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect.