The Collaborative International Dictionary of English v.0.48:

Topple \Top"ple\, v. i. [imp. & p. p. Toppled; p. pr. & vb. n.
   Toppling.] [From Top summit.]
   To fall forward; to pitch or tumble down.
   [1913 Webster]

         Though castles topple on their warders' heads. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Topple \Top"ple\, v. t.
   To throw down; to overturn.
   [1913 Webster]

         He topple crags from the precipice.      --Longfellow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

topple
    v 1: fall down, as if collapsing; "The tower of the World Trade
         Center tumbled after the plane hit it" [syn: tumble,
         topple]
    2: cause to topple or tumble by pushing [syn: topple,
       tumble, tip]