jroudier said:

Hello, sorry to disturb, but I can't upgrade my fortifications. I can upgrade my tower (from 1 to 2), but not doors or walls... I have drachmas for walls (233) so I don't understand. THX !!!

Archon, You can only destroy gate and then build new one on place of the old one.. I guess you discovered fortifications lvl2, if you are able to upgrade tower from lvl1 to lvl2. 
On right side of game you will see ''edit'' button (looks like mouse arrow; under that you have oprtion zoom in/out) press Edit, go to your wall/gate..click.. you will see option to move and destroy (gate/doors)  and you will see move,destroy and UPGRADE option (for walls). After you destroy or move.. you can build new wall/gate.
 If you are still not allowed to do that, please submit your request/problem or bug-report to Plarium Support so that Plarium team could investigate these issues and better assist you in resolving them.