Eel traps?

When I was a kid, my grandparens lived in the middle of a country estate and not far away was a most beautiful lake and walking round it we pased "the Eel Trap".

It seemed to be a brick structure and was quite deep but how did it work?

Was what was left there the actual trap or was it some place where wicker traps of some sort were placed?

Anyone know how it might have operated?