You have now probably run out of ammo which means the next part will be a bit hard. Go through the locked (brass) door, get noticed by the two guards down the corridor and wait for them in the little passage. Kill them one by one with your axe. Then take care of the one hiding around the corner.

Let the guard in this little room notice you past the wooden barrel. Kill him when he comes through the locked (bronze) door, get in there and take the gold key. If you didn't get to kill the guard in the doorway make sure you get out before the door closes.

Go through the upper locked (gold) door, take the health chalices and open the scret doors blocking the retreat. Bring down the armour and take the cross, the silver key (for the door nearby), the plasma ammo and the 1-up. Leave the latter for later if you don't need it now.

Continue.