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.
Illustrated by
Throne of Eldraine#388RareEnglishLegality
Rulings
Oct 4, 2019
Although the common lands have basic land types, they aren't basic lands.
Oct 4, 2019
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.
Oct 4, 2019
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.