It seemed to me that beauty was like the summit of a mountain: once you reach it, there's nothing to do but come back down.
Literal
Beauty [quotative-というのは] mountain [possessive-の] summit [to-に] resembling [to-ように] me [to-には] was-thought. There [to-に] arriving [if-てしまえば] also descending other [to-に] nothing doing [subject-が] not-exist [copula-のである].
An extended philosophical metaphor about the transience of beauty. 似ている ('resembling') draws the analogy. 着いてしまえば uses てしまう + ば conditional for 'once you arrive (and it's done).' のである closes with the explanatory formal pattern. Literary and reflective.