Whether it matters a lot or not, I'd say "the foot (most likely)/base (if you mean its shape)/footing (The least likely option for me) of a tower". Otherwise, it feels like there's more than one.
"A LONELY WHITE TOWER stands at the very edge of the cliff. Its footing is sunk into the water". By the way, what do you think about "sunk into the water"? May be, "submerged into the water", meaning that this tower is built like that, in the middle of the river falling down the cliff.
"A LONELY WHITE TOWER stands at the very edge of the cliff. Its footing is sunk into the water". By the way, what do you think about "sunk into the water"? May be, "submerged into the water", meaning that this tower is built like that, in the middle of the river falling down the cliff.
I wouldn't use "submerged" here as "submerged" only indicates that the footing is under the water. Footings are dug into rock or soil. Also, one can't really sink a footing into the water, as water wouldn't provide any support for the footings. I would opt for something like this, "...its footing is sunk deep into the rock under the water".