The Dragon Lights
A carved obsidian tower broods on basalt cliffs, lava lighting it from below and spilling from its niches in overstated cliche. The effect is somewhat lessened by the bright sunny day. Climbing the narrow cliff faces, and the winding, goat-thin, paths is no problem for seasoned adventurers like yourselves.
You approach from the south and find an outcropping to camp behind, pausing for lunch and to study the keep.
As adventurers, you know that your goal is likely to be at the top of the tower, or if not your goal, than your greatest threat. You start there. The only windows you can spot are arrow slits, too narrow for even a halfling to squeeze through, and downspouts, full to the brim with magma. Someone mentions going through the wall, either by magic or engineering. You consider this for a few moments, before pointing out that the flyer would have to be magically protected against the heat of the lava, drifting in hazy, lethal waves. Not an impossible task by any means, but perhaps impractical to get the whole party in. Still, an option.
Next you examine the bottom of the tower, looking for entrances besides the obvious front door. The lower levels of the keep don’t even have arrow slits, only smooth stone, polished enough for you to see your own reflection. Either side of the front door there are lava outflows with enough space that a flying character could get through, if they were careful and protected from the heat.