"During this passage, Felton related everything to Milady--how, instead of going to London, he had chartered the little vessel; how he had returned; how he had scaled the wall by fastening cramps in the interstices of the stones, as he ascended, to give him foothold; and how, when he had reached the bars, he fastened his ladder. Milady knew the rest. "

Again the translator seems to have made a poor word choice. I find no definition that fits well.

cramp 1
n.
5ME crampe < OFr, bent, twisted < Frank *kramp; akin to MDu & MLowG krampe: for IE base see CRADLE6
1 a sudden, painful, involuntary contraction of a muscle or muscles from chill, strain, etc.
2 partial local paralysis, as from excessive use of muscles
3 [usually pl.] abdominal or uterine spasms and pain
vt.
to cause a cramp or cramps in: often in the passive

cramp 2
n.
5MDu krampe, lit., bent in, hence anything bent in; akin to prec.6
1 a metal bar bent to form a right angle at each end, for holding together blocks of stone, timbers, etc.: also called cramp iron
2 a device for clasping or fastening things together; a clamp
3 anything that confines or hampers
4 a cramped condition or part
vt.
1 to fasten with or as with a cramp
2 to confine; hamper; restrain
3 to turn (the front wheels of an automobile, etc.) sharply
adj.
CRAMPED
cramp one‘s style [Slang] to hamper one‘s usual skill or confidence in doing something

Perhaps the mountain climbing device called "piton" would be closer.Except I doubt there was that kind of mountain climbing at the time of the story.
piton
n.,
pl. 3tons# 73t9nz#; 3tbn$8 5Fr < MFr, a spike, pointed object, akin to OIt pizza, a point6 a metal spike with an eye to which a rope can be secured: it is driven into rock or ice for support in mountain climbing