How do you unlocked the hallowed tower on Pokemon diamond?

Answer 1

You don't unlock the hallowed tower but you can use it. I think you have to of beaten the elite four before you do this. If you talk to the karate guy to the left of Hearthrome City (Route 208), beside the honey tree, he will give you an odd keystone. Then if you go back to the tower and press A you will ba asked to place the keystone into the tower. Then you go underground and talk to 32 people (talking to shop owners doesn't count you need to have a friend to meet underground, you also can't ust keep talking to them, you have to go into your secret base, close the door, then come back out and talk again). Once you have done this go back to the tower and press A. Then a Spiritomb, at level 25, will appear. You can then catch, kill or run from it.

Hope this helps.

P.s The hallowed tower is south of Solaceon Town. If you go straight down then before you turn left to cross the water go right instead. It's right in the corner and if you go up to it without the keystone and press A it says "There is a broken, crumbling tower made of stone..."