|"Uh, I think they saw us coming."|
An impressive number of classic adventures are actually stronghold raids. The first full adventure from the supplements, Blackmoor's Temple of the Frog. The Giants and most of the Drow series. The Slaver cycle. Even Castle Ravenloft, although its mood is very different.
Quick break for a definition. The difference between a stronghold and a ruin is that the walls of the stronghold enclose a nominally unified fighting group. Sneaking through, avoiding raising the alarm, isolating the different groups, thus becomes part of the adventure. A ruin may have sub-areas held by organized groups, but either they are working against each other, don't care about each other, or they are but a nugget within a larger disorganization. So, adventures like the Village of Hommlet's moathouse or the Keep on the Borderland's Caves of Chaos don't really count as strongholds, even though they have stronghold-like areas.
True strongholds are challenging, and you'll notice they were all written for medium-to high-level characters. They are crafted to overwhelm players who come without subterfuge or tactics. In fact, if the importance of strongholds in gaming has faded, this may be because the hobby has drifted apart from its wargame roots. Early D&D grew, via Chainmail, from miniatures wargaming scenarios involving sieges or spying against organized opposition, like Bodenburg and Braunstein.
The ultimate proof of the importance of strongholds comes from the "win" condition of the game, right through AD&D: get together enough men, moolah and mojo to build your own. The victorious player ascends to the Dungeon Master's throne, using the iconic graph paper not just to snail-creep a copy of someone else's dungeon, but to plan and build a stronghold and delvings of their own. Some old-school revival games, most notably Adventurer Conqueror King, hold on to this goal. And it's surely no coincidence that ACK's meta-plot of rising through the ranks of an organization by doing their dirty work can lead in turn to more stronghold busting than you might usually see in a modern-day campaign.
As a final example of the yin and yang of strongholds and ruins in gaming, consider the vast and uncompromising amateur PC game, Dwarf Fortress. You can play in two modes. First, dig and maintain a dwarven town complex underground, mining and crafting treasures and defending it from enemies. Then, after it is overrun (near-inevitably) by demons of the magma layer or invading zombie hordes or simply collapses in civil war, play in Adventurer mode within the same world, as a wandering figure bent on exploring its ruined fortresses, defeating their occupiers and looting their wealth. For sooner or later, every stronghold becomes a ruin...