should warn others that captain can fall into a tunnel and die straightaway. put it on wiki thats what tunnel does and not only as a defensive peasant exit tunnel.

Can you prove it? I've never seen this happen... ever.

Yes it has happened but it's rare because by default you cannot place any units on the tunnel when setting up your attack. What must be done is the attacker places his army and afterwords the defender moves the tunnel and, by luck mostly, places it beneath the enemy's captain.

If you are in the English World 1, I can share a report with anyone who wants to see a tunnel kill a captain, it's very funny.
just send me an in-game mail there since I don't check these forums too often

Oh, also as to the placement of units on tunnels, it will only stop you from placing units on the tunnel if you have a report of the castle with the tunnel in that spot, other than that the tunnel is completely invisible to an attacker