Albert Barnes Commentary Esther 5:14

"Then said Zeresh his wife and all his friends unto him, Let a gallows be made fifty cubits high, and in the morning speak thou unto the king that Mordecai may be hanged thereon: then go thou in merrily with the king unto the banquet. And the thing pleased Haman; and he caused the gallows to be made." — Esther 5:14 (ASV)

A gallows, in the ordinary sense, is hardly intended, since hanging was not a Persian punishment. The intention, no doubt, was to crucify (see the Esther 2:23 note) or impale Mordecai; and the stake or cross was to be 75 feet high, to make the punishment more conspicuous.

Speak to the king... - Requests for permission to put persons to death were often made to Persian kings by their near relatives, but only rarely by others.