Disclaimer - I have never designed a tower of my own. But, I have seen lots of tower drawings. My impression is that most of the tower foundations are done with drilled piers.... Though I've never gotten a detailed / technical explanation about why that it.
My guess is that it relates to constructability. They are not likely to need to do any other earthwork for the site, so it is easier just to bring in a machine to drill three piers than it is to dig out the site for a shallow foundation.
The first thing we need is a description of this "tower" and some information about the subsurface conditions. To me, a tower is a tall building, while to others, a tower may be something else.
Without knowing the loads, etc., of course drilled piers and or ground anchors are an effective method of countering uplift considerations. In the former they are good, too for compression loading; the latter need to be included in a suitably sized foundation for the compression loading. Another way to counter the uplift is to provide a large footing under each leg to a size so that the weight of the soil/rock backfill above will be more than the uplift loads. That is what the designer on my current job did - even with rock near the surface, rubble mortared counterweights were used rather than anchors - of course, the choices depend on location, cost of labour and materials vs machine costs, etc. . .
100m towers (be it building or infrasture) will have high overturning forces applied. PIf the structure is heavy and founding material is good, you can get away with proving a mat slab embedded into rock. If the soil conditions are not so good and you need something the hold your lateral system done then pile supported mats are used. Ground anchors are another option.
