Yeah.... I tried a sandbox at Wave 50.... I had a ton of healers and cannons with shields.... it just instantly fell... I think thats why we got doors... We might have to go overboard and develop multiple layers of defense rather than just one... Having to repair the first walls will be difficult though.... maybe Anuke should add a "break time" every 10 waves so we can build up in peace for 3-5 minutes I guess...