。
A stray dog trampled our garden.
Literal
Stray-dog [subj-が] our garden [obj-を] trampled.
踏み荒らす is a compound verb: 踏む ('step on') + 荒らす ('ravage, destroy'). Combining verbs in this way is extremely productive in Japanese and often creates precise meanings — here, 'trampling into ruin.' 荒らす as an auxiliary often adds a sense of destructive thoroughness.