Do I have to remove the old tiling?

You do not have to, provided the original surface is stable and even. It makes sense to keep the old tiles or cladding only if there are no hollow spots, loosened areas, grease, or cracked grout. The advantage is clear: less demolition, less dust, and a quicker renovation. The drawback is that a poorly checked substrate may later cause cracks or detachment. You can find a more detailed procedure in the related article: https://www.design-solution.cz/content/17-betonova-stěrka-na-obklad-dlazbu