。
That theme park closed down last month.
Literal
That theme-park [topic-は] last-month closing-of-the-park [become-になった].
閉園 'closing of (the) park' is a compact sinitic compound formed for specific institutions that end in 園 — 閉園 for parks / kindergartens / zoos, compare 閉校 'school closing,' 閉店 'shop closing,' 閉館 'building/library closing.' The entire ~する form is condensed to a になる pattern — 'become [N]' = '(it) came to be in a state of closing' = '(it) closed.' A neat lesson in how Japanese builds parallel closing-verbs for different institutional categories.