Harvard Square stalwart Grendel’s Den celebrates its 40th anniversary this year. That’s some history, especially as the popular bar has had the same family at the helm for two generations.

In 1971, in a very different, funkier landscape, Sue and Herbert Kuelzer opened their “casual gourmet” restaurant underneath The Troll House store — hence the name. After some years expanding into the upstairs space now occupied by Upstairs on the Square, Grendel’s contracted to its original basement spot, where it remains a cozy, welcoming haunt overseen by the Kuelzer’s daughter, Kari.

The central, circular bar and a big fireplace dominate the room. Grendel’s “come one, come all” ambiance is reflected in a menu that ranges from vegan chili to above-average burgers.

There’s an appealing retro 1970s style about the menu’s inclusion of such things as a big pot of bubbling fondue, a dish popularized in America during that decade; possibly some of the best cheesy potato skins (topped with either chicken or bacon), another ’70s throwback; and their all-in Beggars Banquet meals. Vegetarian options include tender, steamed tofu and Grendel’s signature spinach pie.

Thankfully, the music is usually contemporary:?Airy indie folk gives way to wilder sounds — Jesus and Mary Chain! — as the evening progresses.