TERREPLEIN is a 10 letter word that starts with T.

1. \Terre"plein\, n. (Civ. Engin.)
   An embankment of earth with a broad level top, which is
   sometimes excavated to form a continuation of an elevated
   canal across a valley.
   
   

2. \Terre"plein`\, n. [F., fr. L. terra earth + planus
   even, level, plain.] (Fort.)
   The top, platform, or horizontal surface, of a rampart, on
   which the cannon are placed. See Illust. of {Casemate}.