I see it spelled wrong all the time, and I'm beginning to wonder if we all are knowing the difference.
While it's true you may become altered at your Altar

ALTAR
Please remember the place you kneel is the Altar It can be a table or a cloth on the ground, sometimes it is a location like a shrine indoors or out and used for purposes of a place worthy of being capitalized, like a proper noun, a big "A" though this isn't the norm... altar/Altar
The more important point is this... the second vowel is an "a" this second vowel differentiates the word from the other.
Example:
A space amongst the trees roots became the scared Altar.
*or*
I'm going to set the Altar with my magic tools.
*or*
The Priest aspersed the room with high altar incense.
*or*
The Priestess stood at the Altar and called to the Goddess.
*or*
Our indoor altar is a lovely short table made of mahogany, the outdoor altar is a cloth we lay upon the Earth.
*or*
Sacrament of the Altar is consumed at the end of ritual.
ALTER
The word alter differs like this, it's a thing you become, to change your way of direction, transforming your way of thinking, rearranging the living room or remodeling the house ...alter.
The second vowel is an "e" and the word is never capitalized, unless it is the first word in a sentence.
Example:
Alter the plans and swing by the bank first.
*or*
The meditation was deep and it forever altered his way of behaving.
*or*
We will have to alter that spell and use a different ingredient.
*or*
The actress would slip in to her alter ego to preform.
*or*
An alternative to having the couch against the wall is putting a tall table behind it.
*or*
Smudging the room would alter the feel of the place.
Let's put them together in sentence:
The High Priest and Priestess flanked the Altar. They were skilled at their ability to alter reality in a magic circle. As the sphere was cast it was clear the space between the worlds altered the feel of the place. The group sat in quiet contemplation after the High Priestess wove a meditation that alters the brainwaves in a deep way. One by one the Page brought each member forward to the altar where a prayer to alter ones direction in life was made. Upon completion of the ceremony all declared they had mind altering experiences and felt the energy while at the altar had shifted their focus and had altered the outcome of the ritual.
(how many do you count, how are they different?)
They're similar in one way...In that the place you kneel or put you tools of "transformation" is the place that will transform your way of thinking.

Remember...Altar (table) noun ...alter (change) verb
sorry in advance for being like such an old school marm.
Blessings, Firebird