That the weight of the city and/or clock (depending on whether whatever jiggerypokery you've got going on affect it's weight on the ground as well as on itself) is likely to crush the earth around it into a huge depression? Could affect how far away it's visible.
Yep, it's foundation is not actually connected to the ground around it, it's buried deep in the earth, so I think I'm okay. For the size, your calculator agrees with what I've come up with, around 0.7 degrees. Since the full moon is 0.5 degrees, that's actually larger than it sounds. When I first saw 0.7 degrees as the answer I despaired, but after learning how that compares to the moon it all worked out.
As for the height to be seen above the horizon, that's some handy information. My world ... well, it sidesteps the curvature problem, I'll say.
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