Don't overthink it; it's effectively a free muster, as you described it initially. (Not OP.) And "first tier" refers to the bottom row of the tisch (you can't drill through to the second tier).
Morgan will be along to correct me if I'm wrong, but there you go!
Thanks Alex, your help is much appreciated and I'm reassured I'm doing it right!
While I've got the attention of someone from Mayfair(!) - I recently bought my copy at the UK Games Expo. Watching online run-throughs I've been alerted to the fact that one of the tiles was misprinted and some versions came with a corrective sticker for that tile. Any idea where I could get hold of that sticker?