Yay, you did a good job, thanks for the multiplayer! To be honest, I also planned to make something like that, to play with my friends online. I really liked Battle City in childhood!
Here is the list of things that I would like to be added/changed to make the game better (it's good now, I just want it to be better, because I would play it ):
Fire seems blurry and slightly unfitting, but this game was made with free assets, so that's not important.
Tank should make holes in the wall that are wide enough for it to pass through, no matter where you make that holes (sometimes they are wide, sometimes not). There are no tank/tank collisions and tank sometimes does not rotate correctly (bug). Adding grid (by that I mean, for instance, tank could move only 8*n pixels at a time, and be at certain positions), would help to move the tank around.
Oh, and add audio - tank should make "rrrr" sound.
In short, the more similar to "Expanded Multiplayer Battle City" this game becomes, the better.
Keep up the good work!
Also, adding more power ups/tank types (Catapults maybe?) should be okay.