Community Nucleus (King of the Hill) strategy

General

Note that due to their exploitable nature the extending catwalks and the point itself are stick-proof: stickies and Jarate will simply bounce off them.
Due to the control point's small size, Demomen and Soldiers are very effective at clearing the point. If there are several enemies standing on the point, the Grenade Launcher's splash damage allows a Demoman to kill most of them very quickly.
A good place to hang out after the point is captured, particularly for Heavies, Demomen and Pyros, are the entrances to the two side catwalks leading onto the point. The enemy is usually concentrating straight ahead at your spawn opposite; with this in mind you can catch them in some devastating crossfire if you are clever about it.

The roofs of the surrounding buildings are open for Soldiers and serve as a great spot to rain rockets down from.

Pyros work well around the lower areas of the catwalk as they can easily poof enemies into the pit or ambush low health enemies from getting the medium healthpack.

Engineers will have a tough time on Nucleus. Sentry Guns erected to overlook the point will be easily taken out by Snipers and Soldiers from the other side. You can try setting up a "surprise" Sentry Gun on the lower catwalks, or alternatively try setting up in the upper balcony area with the medium ammo box to deny that area to the enemy. Use of the Wrangler is effective here, due to the wide, open areas.
Gunslinger-wielding Engineers can exploit the major fight for the central point - due to the concentrated size of the map - to slip by enemy defenses and place Mini-Sentry Guns around the map, effectively being able to flank the enemy forces. It's also a good strategy to place a Mini-sentry below the point, on the Health Kit, in order to surprise both injured enemies looking for health and players on the bridges.

Snipers can be deadly on this map, owing to the unobstructed sightlines available from every side onto the point. For this reason, it is wise to run flanking Scouts/Spies and counter-Snipers to allow your Soldiers, Heavies and Medics to get onto the point without being killed. Be sensible about this; too many Snipers and not enough combat classes and the point itself will go uncaptured.