Castle Garenbrig
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.
Without Garenbrig's strength, the realm would succumb to the wilds.
Adam Paquette
Throne of Eldraine (ELD)
• #240Rulings
Although the common lands have basic land types, they aren't basic lands.
(wotc • 2019-10-04)
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.
(wotc • 2019-10-04)
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.
(wotc • 2019-10-04)
Rulings
Although the common lands have basic land types, they aren't basic lands.
(wotc • 2019-10-04)
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.
(wotc • 2019-10-04)
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.
(wotc • 2019-10-04)