Ancient Ziggurat

The trade here is total color access in exchange for a hard creature-only restriction, and the restriction is what makes the deal so generous. Most rainbow fixing of the City of Brass school taxes you with life loss to discourage greedy bases; this one taxes you with relevance instead. The mana it produces cannot pay for a removal spell, a counter, a planeswalker, or a noncreature artifact, and it cannot pay for an activated ability either, even one that makes or reanimates a creature: the clause attaches to the act of casting a creature spell and nothing else. That confines the land to decks whose game plan is overwhelmingly bodies on the board, which is precisely where five-color tribal and aggressive creature strategies live. For those decks the constraint costs almost nothing, because they were going to spend their mana on creatures anyway, and in return they get a painless rainbow source that smooths out the most demanding part of a many-colored curve. The design lesson is that a downside scoped to a single spell type can be invisible to one archetype while making the card unplayable for everyone else, which is exactly how you hand creature decks a perfect dual without giving control the same gift. The wrinkle worth flagging is that the restriction is about casting, not about the creature ending up on the battlefield: an X creature spell happily takes this mana, but the moment a deck wants a single point open for an instant or an ability, this land stops being a land at all.