Have a look for the splash targets field (for the cannon tower) and make sure only enemy is checked for that field.
btw, thank you for making sure you fix this up. The number of tds I've played where the map-maker hasn't done this tends to scare me at times. Everyone builds the powerful towers at the end only to find it ends up blowing up all their other towers when firing.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.