The story is probably The Outsider (1921) by Howard P. Lovecraft, which definitely has aspects of Gothic fiction.
The story is available on Wikisource. Below are a few excerpts:
In the dank twilight I climbed the worn and aged stone stairs till I reached the level where they ceased, and thereafter clung perilously to small footholds leading upward. (...) Then came a deadly circuit of the tower, clinging to whatever holds the slimy wall could give; till finally my testing hand found the barrier yielding, and I turned upward again, pushing the slab or door with my head as I used both hands in my fearful ascent. There was no light revealed above, and as my hands went higher I knew that my climb was for the nonce ended; since the slab was the trapdoor of an aperture leading to a level stone surface of greater circumference than the lower tower, no doubt the floor of some lofty and capacious observation chamber.
Believing I was now at prodigious height, far above the accursed branches of the wood, I dragged myself up from the floor and fumbled about for windows, that I might look for the first time upon the sky, and the moon and stars of which I had read. The sight itself was as simple as it was stupefying, for it was merely this: instead of a dizzying prospect of treetops seen from a lofty eminence, there stretched around me on the level through the grating nothing less than the solid ground, decked and diversified by marble slabs and columns, and overshadowed by an ancient stone church, whose ruined spire gleamed spectrally in the moonlight. (...)
The sight itself was as simple as it was stupefying, for it was merely this: instead of a dizzying prospect of treetops seen from a lofty eminence, there stretched around me on the level through the grating nothing less than the solid ground, decked and diversified by marble slabs and columns, and overshadowed by an ancient stone church, whose ruined spire gleamed spectrally in the moonlight.
