Central Tower is in the middle of Hyrule, south of Hyrule Castle. This page gives advice for activating it.

Activating Central Tower

Central Tower is just to the left of Hyrule Castle when viewed from Great Plateau Tower. You can float toward it from the Great Plateau Tower, then carefully approach the area on foot. There are Guardian Stalkers in the area, and there are Decayed Guardians around the tower itself.

You can try to just kill the Decayed Guardians, but if you are not confident in your ability to do so, then try to activate the Kaam Ya'tak Shrine south of the tower before you approach the tower itself. The shrine is next to a hill. Climb that hill and go to the apple tree on top of the hill. Then float to the tower from the highest point of that hill and you should be able to float all the way to the tower itself, grabbing onto it to the right of a platform that will block the aiming laser of the Decayed Guardian nearby.

From there, you will need to carefully climb the tower in such a way that the Decayed Guardians can't aim at you. As you climb, you should try to jump up to the next platform so the Guardian doesn't have time to finish aiming before you reach the safety of the platform. Make sure that once you are on a platform, you are standing on it in such a way that it blocks the Guardians' beams.