Tumbar means 'to knock down' or 'to tear down'. While it can refer to physically demolishing something, it often carries a stronger, more decisive connotation than simply 'to fall'.
In this song, the singer uses tumbar to express his intention to completely dismantle the house he built for his lost love. It's a powerful verb that conveys not just destruction, but also a desperate attempt to erase memories and move on from a painful past.
This song is all about getting over a breakup, and it's got a pretty intense way of showing it! The singer is ready to tear down the little house he built, stone by stone, for his lost love. He asks, "Para que la quiero / Si ya te perdí" [Why do I want it / If I've already lost you?].
He hopes that with every stone he knocks down from the house, "Quiera dios que se derrumbe / Adentro de mi alma / Tu imagen también" [God grant that your image / Inside my soul / Also crumbles]. It's a powerful image of trying to erase someone from your memory by destroying something physical. He doesn't want her "fantasma" [ghost] or "recuerdo" [memory] lingering around. He's ready to start fresh, hoping that by tomorrow, as he begins to "tumbar la casita" [tear down the little house], he'll also start to forget her.