What are the main features that the narrator observe ?

It's difficult to match with your lines because different stories are printed differently. Did you mean the description of the house?

Lines 94 to 98 are

Beyond this indication of extensive decay however the fabric gave little token of instability. Perhaps the eye of a scrutinizing observer might have discovered a barely perceptible fissure which extending from the roof of the building in front made its way down the wall in a zigzag direction until it became lost in the sullen waters of the tarn.

The narrator observes that there are small cracks(fissures) in the very foundation of the walls and house. This of course foreshadows both Roderick's demise as well as the house itself.