Do you mean the battle tower? If you got to the locked door near the teleporter shrine, head back down to where the path branches, and take the other direction. Once you kill the bandits in that area you should be able to open the door there.
Lovis' tower in Broken Valley has some ladders near the base you can climb up to get in the back way. From the front gate you should be able to edge around the left side and jump over into that section. Keep an eye out for levers to open the various gates, sometimes on the floor, sometimes on ledges you need to jump up to.
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