We had a 2nd or 3rd level party going into the city sewers, I played a dwarf Cleric. Turns out the bad guys were goblins that had done a decent job of fortifying themselves. After we breezed through the initial defenses there was a 10ft deep bit of water, a 10ft climb, to get past the 2nd layer of defenses that were built like a small fort. We decided to pull back and think of something other than just charging, the goblins weren't going anywhere. It was a long time ago so I am surely forgetting a lot of the little details, but basically we went back up to town, got a tower shield, a boat just big enough to carry us all that we could still fit down the sewers and carry manually, and a 20ft ladder. Basically did a little siege of the goblin fort, and it worked pretty damn well. Had a mage type and someone with a bow stay back and plink at range, the rest of us took the boat over under cover of the tower shield, then my dwarf went up over the top getting pounded on pretty good. But we carried it easily once we got 2 guys up over the little wall. We were very proud of ourselves.
Other time i was playing an elven enchanter, we were around 13th or so, also still 3.5E, and there was a bad guy fort in the woods. I used dominate person, alter self, and detect thoughts to kidnap and then impersonate a lieutenant of the enemy for a full day and got all the information we needed to gain access to the camp, the guard rotations, and who to take out and in what order. We snuck back in at night, managed to assassinate all the most dangerous people while they were not at all ready for a fight, then basically route the rest of the fort as we still had pretty much all our resources, spells, and HP left. A smashing victory.