ok,played with the mines last night, seems i can walk on them,seems i can build roads over them and farms,they do not dissapear on 1st killing,they seem to randomly stay for more than 1 kill,had one stay for 4 turns,killing each time also,they go off without being set if left in the city and you hit the button for missing 1 turn,if a spy appears in the city it goes off killing the spy but i was not set,its happened many times for sure so far,just tinkering with them and thats what i see so far,thought they HAD to be set 1st,also thought they were good for only 1 bang