There really are not that many rules:
"The U.S. Constitution states that the president must:
Be at least 35 years old
Be a natural-born citizen of the United States
Have been a resident of the United States for 14 years
Anyone who meets these requirements can declare their candidacy for president. Once a candidate raises or spends more than $5,000 for their campaign, they must register with the Federal Election Commission. That includes naming a principal campaign committee to raise and spend campaign funds."
The 14th amendment to the Constitution (after the Civil War) added this (parts in bold is what got him kicked off the ballot in Colorado):
Section 3.
No person shall be a Senator or Representative in Congress, or elector of President and Vice President, or hold any office, civil or military, under the United States, or under any state, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any state legislature, or as an executive or judicial officer of any state, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof. But Congress may by a vote of two-thirds of each House, remove such disability.