Crossing the Upper Swamp

The change in terrain is noticeable climbing up out of the festering pools and meres were hills meet plain, as is the shift in senses mentioned yesterday. Whatever brief burst of hope that comes over the players quickly fades as they see the rising expanse before them. There is a high ridge running across the swamp, dividing upper and lower regions. Cresting it gives the players a view ahead of the valleys and spines of lava breaking up the festering pits and pockets of slime and ooze that are now mixing with hot mud and lava. It is a despairing view ahead that quickly envelops all thoughts as they descend the far side of the ridge back into the molten quagmire that is the hillside ahead.
Soon the path drops into a dark ravine running southwest in a mostly straight run. The rough lava walls have worn smooth with decades of erosion, but still jut out awkwardly in areas, making passage difficult at times. More than you've noticed through the swamp so far, you can move fairly quickly here for a couple of miles, weaving among the veins of hardened lava, occasionally jumping over swells or through breaks into a parallel vein.
After following along this way for another mile, you are struck by a formation rising up out of the surrounding terrain about a half mile away to the left of the direction your paths are leading you (perception DC15 will note movement near the pinnacle of the rock). If anyone pays particular attention, have them roll a perception check (DC12 to notice a very faint trace of smoke rising from near the top). Oddly, there seems to be some indication on the map you have been following in the same general area, but it is unclear on the map what is being referenced. Imogene initially took it to be a landmark symbol denoting the tower of rock.
Once they have noticed the oddities, the party can search for a route over to it (survival DC12 to find). After a few hundred yards out of the ravine, the path actually becomes more pronounced, rising out of the morass around it and hugging the far side of the rock out of sight of the main trails you have been following through the swamp.
As the party begin circling the rock, they can more clearly see a thin column of smoke rising from a pair of windows high in the tower. Clearly something is living inside some kind of chamber at the top of the tower (survival DC18 to notice the traces of passage are humanoid (human, in fact)).
At this point, have the players note their order of movement, and any special measures they are taking to approach (stealth, active perception, arcana checks or magical detection, etc.). The tower is held by a half-mad cleric, paranoid from years in his surroundings. He has many traps, alarms, etc. in place around the hillside to protect him from attack, and warn him of any approach.
Nalen Tassler is a loner and self-proclaimed bastion of defense against the evil breeding in the swamp. He has come here and remained to wage war on the spread of the disease via the slaad. He believes them to be abominations, and is trying to prevent them spreading any further out of the murk they inhabit. So far, they are content to remain where they are, finding little resistance to getting what they want within reach of their community. They largely ignore Nalen as a nuisance.
Nalen has studied slaadi culture for most of his adult life, and works to protect the world from them. He has scrolls and potions and spells at his disposal in addition to knowledge, and though mad, will share them if the players win his confidence in any way.