If you cast a spell for another cost "rather than pay its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Spark Harvest, those must be paid to cast the card.

Bolas's Citadel may be one of the permanents you sacrifice to activate its last ability.

You must follow the normal timing permissions and restrictions of the cards you play from your library.

Bolas's Citadel lets you look at the top card of your library whenever you want (with one restriction—see below), even if you don't have priority. This action doesn't use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.

If a spell has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

You can play a land card from the top of your library only if you have available land plays remaining.

If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, you can't look at the new top card until you finish doing so. This means that if you cast the top card of your library, you can't look at the next one until you're done paying for that spell.

In a Two-Headed Giant game, the last ability of Bolas's Citadel causes the opposing team to lose 20 life.