First: With tectonic activity and geologic timescales, you’ll see possibly the entire continent migrate to a new climate, before full turnover. This could mean glaciation, mountain ranges, canyons and river or dense jungle. How it looks now, it might not look in one million years, at all. If there’s wildlife, no one will care about signage if the wildlife is healthy.
Second: Archeological expeditions will try to explore the architecture, and some will die, before they figure it out. So, with explorers in mind, the spikes are a good landmark to raise general awareness that this unnatural structure exists, but, realistically, one has to expect attrition before any interested party gains full understanding, and thus a means of wayfinding that produces a sort of conceptual mummy’s curse should be engineered with an entrance.
Third: There’s no consideration for intelligent animals of different sizes or scale in proportion to non-human bodies. From crustaceans and mollusks, to dancing bees, to talking dogs or socially regimented elephants and woolly mammoths that communicate with infrasound. What about those guys? Space aliens, you can bet will be fine, but nascent intelligent life that erupts in the wake of an extinct human civilization might need a clue as well...