Millwalls old football ground was called The Den, so named because if was the home of the Lions. It was situated on the Isle of Dogs and rightly had the reputation as one of the most inhospitable football grounds for away supporters to visit, it was in actual fact closed by the FA on no fewer than 5 occassions, 1920, 34, 47,50 and 78 !!
It was also closed for a much different reason during the war when German bombs damaged it, causing Millwall to play their football at Charlton, Palace and West Ham. They eventually returned and played there till 1993.
The picture below was taken from a Football Review magazine and shows how the ground looked in the early 1970’s, the text reads as follows;
"Millwalls compact ground has a capacity of 40,000 with seating for 3,480. The record attendance at the ground was set in February 1937 when a 48,672 crowd watched an FA Cup fifth round tie against Derby County. The Dens playing surface measures 112 yards by 74 yards. Inset: Millwall manager Benny Fenton."
Ive also found a link to a video which features some great old images of the ground as it was ...... enjoy.