Spasskaya Tower is the Kremlin's great eastern gate, facing Red Square with the clock and red star that have become part of Moscow's visual shorthand. The tower was built in 1491 and gained its high tented roof in the seventeenth century.
You will see it naturally while visiting Red Square or the Kremlin walls. It is best treated as part of that larger walk, but it deserves a pause of its own for the view down toward St Basil's and the square.