What is "interior crest"

Crest \Crest\ (kr[e^]st), n. [OF. creste, F. cr[^e]te, L. crista.]

1. A tuft, or other excrescence or natural ornament, growing on an animal's head; the comb of a cock; the swelling on the head of a serpent; the lengthened feathers of the crown or nape of bird, etc.
   --Darwin.

   [Attack] his rising crest, and drive the serpent back.
   --C. Pitt.

2. The plume of feathers, or other decoration, worn on a helmet; the distinctive ornament of a helmet, indicating the rank of the wearer; hence, also, the helmet.

   Stooping low his lofty crest.
   --Sir W. Scott.

   And on his head there stood upright A crest, in token of a knight.
   --Gower.

3. (Her.) A bearing worn, not upon the shield, but usually above it, or separately as an ornament for plate, liveries, and the like. It is a relic of the ancient cognizance. See Cognizance,

4. 4. The upper curve of a horse's neck.

   Throwing the base thong from his bending crest.
   --Shak.

5. The ridge or top of a wave.

   Like wave with crest of sparkling foam.
   --Sir W. Scott.

6. The summit of a hill or mountain ridge.

7. The helm or head, as typical of a high spirit; pride; courage.

   Now the time is come That France must vail her lofty plumed crest.
   --Shak.

8. (Arch.) The ornamental finishing which surmounts the ridge of a roof, canopy, etc.

   The finials of gables and pinnacles are sometimes called crests.
   --Parker.

9. (Engin.) The top line of a slope or embankment.

   Crest tile, a tile made to cover the ridge of a roof, fitting upon it like a saddle.

   Interior crest (Fort.), the highest line of the parapet.

n. (context fortifications English) The highest line of the parapet.