i walk through stairs

i was building my dog a dog house when i had to stairs facing me and forwards like you were to walk up them and to stairs upside down above them where they made like an arch. i dont know if this is fore all stairs but i was using acacia for the bottom ones and tiled deepslate stairs for the top stairs that were making an arch. i was able to wal inbetween the arch like the inner part of the stairs werent there.