the following applies to towers as tesla & arrow-tower:
since they drop down if placed onto a bridge and thereafter removing the bridge they stand on it's possible to place them where already other items are.
accidently removed the bridge and tesla shares place with a stockpile:

purposely multiple defense setup (arrow-towers & tesla share the place)

accidently fell down because i built another bridge underneath and the bridge where tesla intended to stand on changes to piller-layout (see center-tesla above stockpile) now shares place with the bridge where it was supposed to stand on

for towers i'd suggest to lock the block where they stand on - or is the intention to be useable like miners lamps and to protect the mining-area?
Also if want to remove a level-3-tesla at some location where it's not needed any more to rebuild it at another spot: not possible since it does only return the resources for a level-2-tesla on deconstruction.