The impeachment process is a criminal process under the Constitution [high crimes and misdemeanors] but as the constitution makes clear in this one single proceeding the Congress has jurisdiction rather than the federal judiciary.
As such due process protections apply including right to be informed of the charges against him and the right to subpoena witnesses. Due process means that the Federal Rules of Evidence (which are Congressionally approved) apply.
Yup. This impeachment, has been unconstitutional. And if Lady Graham has his way, it will continue to be unconstitutional.