Tumble-down is an adjective that is often used to describe something that is in a state of disrepair, dilapidated or falling apart. Its antonyms, or opposite words, would describe something that is well-maintained, sturdy, and intact. Such words include "strong," "solid," "stable," "secure," "firm," "steady," "sound," and "robust." These words suggest a sense of durability and reliability, rather than fragility and instability. An example of an antonym for "tumble-down" would be "well-built" to describe a house that is sturdy and not in danger of collapse.