The suggestions–offered by Garden Tower Project–were read and documented (for reference). Strangely (my fault?), I couldn’t locate casters at Lowe’s, and casters at Tractor Supply were “too expensive” (imho). Furthermore, at Tractor Supply, I wasn’t provided information about the composition of the wheel. At Garden Tower Project, polyolefine was recommended. I hasten to say that the casters I ordered are probably much heavier than I need. But, this old lady wants to effortlessly navigate her Tower!!
I searched the Internet (far and wide) for a “drum dolly” (none locally at Lowe’s or Tractor Supply). Drum dollies are widely available but I decided, instead, to spend extra dollars on exceptional casters. They will be attached to an unattractive–but sturdy–platform of two 2”x12” planks.
Some bolts will be purchased–to secure the Tower–but I speculate my total expense will be less than $60.00.