FO OF A DC9-50 DSNDED BELOW ASSIGNED ALT FOR TRANSITION FOR APCH DUE TO MANY DISTRS AND A LAST MIN CHANGE IN TYPE OF APCH. THE CTLR INTERVENED AND DIRECTED RPTRS BACK ON ASSIGNED ALT.

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ON APCH TO RWY 12L AT STL; WE WERE GIVEN AN ALT OF 4500 FT FOR LEVELOFF. DURING A TURN FROM BASE TO FINAL; WE WERE ALSO GIVEN AN EXPECT LDA DME RWY 12L. THIS WAS NOT WHAT WE WERE EXPECTING. WE PLANNED A VISUAL WITH ILS BACKUP; AND HAD THOSE PLATES OUT. BOTH THE COPLT AND I HAD TO GET THE NEW PLATES; AND WHILE DOING THAT; CTLR WAS GIVING US TFC AND RWY INFO. I SAW THE RWY BUT COULDN'T FIND TFC. CTLR KEPT GIVING TFC INFO AND SAID 'I CAN'T DO ANYTHING UNTIL YOU HAVE THE TFC.' SO I WAS LOOKING WHILE THE FO WAS FLYING. WHILE THIS WAS ALL GOING ON; WE WENT BELOW OUR 4500 FT. ALT WARNING WENT OFF AT 4200 FT AND WE RECOVERED. OTHER THAN THE OBVIOUS FACT THAT WE MISSED A LEVELOFF; I WILL SAY WE WERE HELPED BY NUMEROUS OTHER EVENTS. FIRST; WE WERE PUT IN HOLDING ON THE ARR DUE TO AN EMER AT THE FIELD. THIS PUT FUEL ON MY MIND. WE WERE THEN TOLD TO EXPECT A 15-20 MI FINAL AND WE WERE DSNDING ACCORDINGLY. WE WERE THEN TURNED AT 10-11 MI AND SWITCHED TO LDA-DME RWY 12. THIS PUT US BEHIND. WHILE TRYING TO CATCH UP; WE WERE TOLD OF THIS TFC; SO I WAS SPENDING MOST OF MY TIME LOOKING OUT FOR IT. THE COMBINATION OF ALL THOSE FACTORS RESULTED IN US GOING BELOW 4500 FT ASSIGNED ALT. PERFORMANCE WISE; IT WAS THE LAST LEG OF THE DAY AND WE HAD A SHORT LAYOVER WITH AN XA00 DEP THE NEXT DAY; HOWEVER; I DO NOT FEEL THAT WAS REALLY A FACTOR. WE JUST SIMPLY GOT OVERLOADED AND MISSED AN ALT.