After a chimeric idol attacked them, the Keldons smashed all unfamiliar statues.

Chimeric Idol

{0}: Tap all lands you control. This artifact becomes a 3/3 Turtle artifact creature until end of turn.

Vintage Masters

{0}: Tap all lands you control. Chimeric Idol becomes a 3/3 Turtle artifact creature until end of turn.

Prophecy

o0: Tap all lands you control. Chimeric Idol becomes a 3/3 artifact creature until end of turn.

Your lands are tapped as part of the effect, not part of the cost.

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.