What are the low points of this captcha method

I use this captcha method in all my forms on many sites, the main premise is that I have a text field that is hidden by css, if the server-side code determines that there is any content in this field, the form is not completed .

Client side:

<li id="li-cap"><label>Leave this field blank</label><input type="text" maxlength="30" id="cap" name="cap" /></li>

Css:

#li-cap{display:none}

Server side psuedo code:

if(!nullOrEmpty(input#cap))
{
    return post back to form with error
}
else
{
    process form
}

it ignores any client check at the moment

I use this form of captcha (I believe that it has a name, I can’t remember, although I think it starts with p) for a while, and I see many different types of captcha: mathematical sums, random letters in images, questions.

, -. , ? ?