Follow the steps to make a cool trap with an engineer with oil(must be done in editor).
Make a wooden gatehouse(and block/remove stairs) and an engineer with oil.
Tell the engineer to go trough the gate, and close it before he can get out again
Destroy your gatehouse(don`t worry the engineer are still there, he is just invisible)
Locate your invisible engineer, and DO NOT move him(this will make him visible again). set him on agressive or defensive stance.
Wait tell an enemy to go near you engineer...
The engineer will pour his oil on the enemy(s)
and burn them. Its better to use enemy engineers in this trick, your will be shot at if they are in range.
You can use this cool effeckt to make such things as fountains or volcanos that burn enemys when they came near.
(The download is a map where you can see this trick, with a little extra pitch)