first before you should even think about going in, you have to get your national dex and beat the elite four. then you go to the temple and the person at the front at first doesn't let you in, but then a girl comes and talks to the lady and then you can go in.
in the temple is regi gigas. ( i don't know how to spell it so sorry if i did) you will need regi rock, regi steel, and regi ice to get him.
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To get to the snowpoint city temple you have to beat the Pokemon legue champion first you also need to have regirock, regice and registeel but to have these Pokemon you will have to trade from ruby, emerald or sapphire
ask my brother, Jeffrey
You need to complete the Pokemon league
Snowpoint city at the top of shinno Pokemon daimond & Pearl
They are near Snowpoint City in Pokemon Diamond Pearl.