Ziggurat

There are floors to complete. When you finish the last one this will pop.

Some tips for getting to the end:

1) Set the game on easy.
2) Always complete every room on a floor. The extra XP will be very helpful later in the game.
3) Try to find the secret room on each floor. Look for large cracks in wall tiles. Some are more obvious than others. Use logic when looking at the map. If there is no space for a room, there is no room. Also doors seem to be evenly spaced and all secret rooms I found were where a door would be. Also, it takes quite a few shots from your wand to break the wall.
4) If it's not too risky, try to collect the drops as quickly as possible, they don't stick around long. This means running toward enemies at times to grab items.
5) When possible, try to keep an AoE item for when things get crowded. Fire bomb or spike bombs are great.
6) The rest is standard FPS affair. Dodge hits, get kills quickly etc. etc.

I hope this helps. Game on achievement hunters! =D