How do dual lands and their mana work? [duplicate]

Question

Certain lands, such as Coastal Tower, state that they can add two types of land, but when does that happen? When I cast it, it enters the battlefield as just one land, right? So does it add the additional land when tapped?

Answer 1

Lands and mana are not the same thing. Lands (generally) have abilities that let them tap to produce mana. For example, a basic Island can tap to produce one blue mana, and a basic Plains can tap to produce one white mana. (On modern printings, this is implicit, but on old ones it was actually written out on the card.) The land itself is just an object on the battlefield; it's the mana it produces that you spend to cast spells.

Similarly, some nonbasic lands have abilities that let them produce more than one kind of mana. Your Coastal Tower can tap to produce mana just like a Plains or Island could, except you get to choose: you can produce a blue mana or a white mana. It's sort of like being able to play an Island and a Plains in one card, with the downside that it enters tapped and is not actually an Island or a Plains.

For example, if you want to cast a New Prahv Guildmage, which costs a white mana and a blue mana, and have an Island and a Coastal Tower on the battlefield, you can tap the Island for a blue mana and the Coastal Tower for a white mana. Or if instead you had a Plains and a Coastal Tower, you could tap the Plains for a white mana and the Coastal Tower for a blue mana.

A final note, you don't cast lands. They're not spells. You simply play them.

Answer 2

You don't have a sound understanding of how lands and mana work yet, I'm afraid. I think you haven't read a good explanation of what's going on with them, unless Jefromi's answer did the trick.

I think your mental model is that merely having lands means you can cast spells (e.g. you can cast Herald of Kozilek as soon as you have 1 mountain, 1 island, and 1 of any other land), and therefore dual-color lands must be two types of land — but this is wholly incorrect, so if that's what's going on it's no wonder you're getting confused. You're missing some big parts of the picture, the biggest of which is mana.

Coastal Tower is just a land that sits there. It's never two types of land; it's just Coastal Tower. It doesn't count as an island or a plains and doesn't have to. It has an activated ability that lets you tap it to get one of two kinds of mana (white or blue, your choice) and you can choose differently or the same each time you use that ability. You use that mana (not the land) to cast spells.

I recommend you read the official Quick Start Rules (that's a link to the PDF for them) to get that basic explanation, or the slightly longer 2011 Basic Rules. In particular, you need to read the Quick Start Rules' section on lands (page 4) and casting spells (page 6). The rest will also be a useful read; the quick start rules explain things a bit better than the fold-up sheet distributed in products nowadays.

Mana symbols ≠ land

Lands are used to give you mana (white, blue, black, red, green, or colorless¹), which is a resource or currency you gather and then spend on spells or abilities. Mana doesn't come just from the "types of land" or even just lands: Forest will give you green mana, but so will Blossoming Sands or Woodland Stream, despite neither one being a forest. Creatures like Elvish Mystic or Opaline Unicorn can also add mana to your mana pool, and they are not any kind of land.

When you cast spells, you'll do it by spending the mana you acquired from various sources (lands, creatures, etc). You don't cast spells merely by having lands sitting there: merely having three forests and a Blossoming Sands doesn't all on its own let you cast Glare of Subdual; you actually have to tap them and get the mana from them and use that to cast it. (Which means you don't also get to spend that mana on other spells this turn.)

All of these cards I've mentioned have an activated ability which is what you use to get mana. The 2011 Basic Rules explain activated abilities on page 11. Nowadays basic lands just have that activated ability implied; older basic lands had it written down explicitly.

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1: White, blue, black, red, green, and colorless are represented by the letters {W} {U} {B} {R} {G} {C} in that order. In cards before the Oath of the Gatewatch set (printed in 2016), cards that gave you colorless mana just represented it with a number that looked exactly like the generic component of a mana cost, e.g. {1}.