THE SECRET COW LEVEL
Many fellows have asked how to get to the mythical cow level. After much research into the matter, and through many trials and errors, we have found numerous ways that you can achieve what is impossible and gain entrance into the fabled level. Listed below are those set methods. But as honourable players, we must advise you that we can not be held responsible, if you do try to access the legendary cow level and fail.
The first method involves hitting any one of the cows in town 100 times. If you hit them more or less then 100 the count is nullified so you have to make sure you stop on precisely 100 hits. This opens up a portal behind the church that allows you to go down to the secret cow level. Once down there you are confronted by mutant barn animals - such as shovel-carrying cows, skeletal chickens, food-spitting pigs, two-headed roosters, and a couple of naked farmers thrown in for good measures.
The second method is to buy out Griswold's workshop thus leaving him empty. When you next visit him he suddenly has a scroll, something he never sells, which when thrown from a level other than town opens the portal to the cow level wherever you are. This is much nicer and more handy instead of constantly hitting the cows since the port is thrown right by you for easy access - though once you have thrown it, if you throw a town portal, you lose the cow port so be prepared before going down.
The third method is hidden in the first quest against the Butcher. If you allow yourself to die by the Butcher's hand in his room a scroll is dropped from him while you are restarting in town - you must restart for this to work. You have to go back and kill him in his room by leading him in there or the scroll will disappear when he dies, if killed out of his room. Once again, get prepared and throw the scroll whenever you feel like going down.
The fourth method found was through using the goat shrines/cauldrons. I haven't tried this one yet but those that I trust have assured me that it works. When in Hell you have to hit all of the shrines on levels 13 & 14. Once the last shrine is hit a portal by the pentagram on 15 is enacted leading you down to the secret level. You must throw your own town portal before you can enter the cow level though. Must be some bug in the game. I don't know if this works only for Hell/Hell or for the other difficulties but I play with golden children who only play Hell difficulty so I know it works there.
ABOUT THE COW LEVEL
When you are down in the cow level the creatures tend to come in swarms, at least when I've played down there, so be careful of rushing through. The cow level isn't like any of the other levels. It is dark in the form of a massive barn with only 2 levels to it - the main area and the "hayloft". Each level has a couple of rooms accessed by doors, no open archways, which usually have tons of animals on the other side. They don't attack till you get in there so standing at the door and picking them off one by one doesn't work.
Once the two levels are cleared the final "boss" comes out - a creature I don't want to reveal cause you have to see it to believe it. He is a nasty one. Harder than Lazarus, but not as hard as Diablo. Spells don't work on him cause he's resistant to all so you'll just have to hack him to bits. When he's dead one of the unique items will drop already identified. This is good for all except the Gotterdammerung which a lot of people like to use unidentified just for the armour value.