Source: http://ser.sese.asu.edu/M10/IMAGE_ARCHIVE/CLEAN/0027415.png
This is a dark tower I found on planet Mercury in a NASA archive. Most craters on other planets and moons are about 10-30km across. So I will take the low number 10km and the tower is 1/3 the diameter of the crater making it 3km tall. Now thats a low end estimate. If this unknown crater is 30km across, then this tower is actually 10km tall! The gravity on Mercury is 38% that of Earths, so any building will weigh 62% less on Mercury, compared to here on Earth.
There is also a huge statue of a person praying in the left side of the photograph. This could mean that the tower we see is of religious significance to the aliens species that created it. The tower could also be a docking point for ship too large to land not he planets surface.
Scott C. Waring