Attempt #1
The enemies of the Jews saw this as their last opportunity to prevent the fortification of the city. They knew that once doors were installed in the gates, Jerusalem would become impregnable.
Attempt #2
Sanballat deliberately sent an open, unsealed letter to convey to Nehemiah that the message it contained was already common knowledge.
וגם נביאים העמדת והם שכרו את הנביאים שבעת הצורך יאמרו שנחמיה שכרם לזה לנבא כן וזה מרידה, ועפ"ז ברר לנחמיה שרוצה למרוד, כי העמדה ושכרת נביאים לקרוא עליך שתהיה מלך ביהודה...
He spoke the truth, there were propehts in the city declaring Nehemiah as the king of Jerusalem. Sanballat and his cohorts had hired these false prophets as part of their endeavor to frame Nehemiah as a traitor.
Attempt #3
He suggested that Nehemiah sequester himself in the comparative safety of the Sanctuary; unlike the gates of the city, the Sanctuary gates were fitted with doors.
And then Sanballat and his men would have a claim against me, as I am not a priest and am therefore forbidden from entering the Sanctuary...
Attempt #4
Even after the completion of the construction, even though they saw that it was done with aid from Gd, still, many of the officers of Judea would send letters to Toviah, and the letters from Toviah came back to them with friendship and loyalty.