Abilities and layers

Let's assume that there's a Urborg, Tomb of Yawgmoth on the battlefield, and it's enchanted with a Guardian Zendikon.

Effectively, Urborg will now be like this:

Its full type line will be: "Legendary Land Creature - Swamp Wall". (It's a swamp because of its own ability, and this gives it the ability to tap for one black mana.)

Now let's assume that someone enchants it with Lignify.

Thus it ends up effectively looking like this:

Question: Are all lands swamps in addition to their other types?

One could hastily think that they are not, because Urborg does not have that ability anymore. However, as is usual in this blog, it's not that simple.

Urborg's ability is a type-setting ability, while Lignify's ability is an ability-setting (or in this case an ability-removing) ability. These are evaluated on different layers.

The status of all permanents on the battlefield is evaluated one layer at a time, starting from the first one.

In this situation layers 1 to 3 have nothing special to them. However, when we get to layer 4 (the layer where type setting abilities are evaluated), we see that Urborg has an ability that sets the type of each land to Swamp.

It's important to understand that at this point Lignify's ability has still not been evaluated (because its effect happens on layer 6.) Therefore at this layer Urborg still has the ability "Each land is a Swamp in addition to its other types." This is evaluated now, and therefore it sets the type of all lands to Swamp (including Urborg itself.)

Then, when we evaluate layer 6 we see that Lignify is removing all of Urborg's abilities. Thus it affects Urborg only. (Remember that so-called dependent abilities are evaluated only when they happen on the same layer. Therefore Lignify's ability has no effect on layer 4.)

Also note that even though Urborg is a Swamp due to its own ability, it does not have the ability to tap for one black mana, because Lignify is removing it.

Likewise Lignify is removing the "Defender" ability set by Guardian Zendikon, due to Lignify having a newer timestamp. (Urborg could now freely attack.)