
Land
This land enters tapped unless you control a Forest.
: Add
.
,
: Add six
. Spend this mana only to cast creature spells or activate abilities of creatures.

Land that produces large bursts of mana for green-heavy creature decks.

Land tapping for green mana scaling with creatures, premium ramp.

Land that multiplies mana output for big plays.

Creature tapping for green mana scaling with devotion.

Doubles your land mana for explosive green ramp.

Produces massive green mana for casting creatures.
The mana generated by Castle Garenbrig's last ability can't be spent to activate abilities of creature cards that aren't on the battlefield.
Once the common lands (such as Mystic Sanctuary) enter the battlefield tapped, there's no way to untap them with a spell or ability to make their last ability trigger.
Although the common lands have basic land types, they aren't basic lands.