Montagnon de Carsabel and the banks of the Lios

Park in the village square, Place des Écoles, near the church.
The route is not marked. The route starts in the village centre, in the streets. Use the street signs to find your way.

(S/E) Cross the square to reach Rue de la Place (you couldn't make it up!) to the south.

(1) At the first crossroads, continue straight on Rue du Pradal. 100 metres further on, take the dirt track on the right to cross the small bridge towards the park. Enter the municipal park. Turn immediately right onto the lawn, through the trees, to reach the elegant renovated arbour. Then continue downhill towards the River Sios and reach the gate of a large house. On the right, you will see a superb 18th or 19th century dovecote, which is very well maintained.
Opposite the gate, take the path behind you that runs alongside the river in the park. Continue along this path through the isolated trees and forest for about 600 metres in a south-south-easterly direction until you reach a covered footbridge crossing the river on your right. Do not cross, but continue along the path on your left until you leave the park to the east and reach the Chemin de Villeneuve road.
Turn right onto the road, which quickly turns into a rural service track. On the left, leave the property in the clearing and, at the fence, turn left onto the track that climbs towards the forest and the slopes of Carsabel for 140 metres.

(2) Leave the track and turn left onto a narrow path made by a forestry machine (such as a tracked loader) that goes into the forest, directly up the slope. The path makes two hairpin bends on this very steep slope. It is marked like a trail (orange paint marks on the trees from time to time). You will reach a sort of plateau. A few dozen metres to the left, you can reach the northern peak of Les Serres, Cap des Serres. Otherwise, continue south-southeast on this same fairly narrow forest path, which quickly turns into a wider track (marked with a solid black line on the IGN map). You will reach a fork in the path.

(3) Turn left onto a new narrow forest path. The path reaches an old clearing where the forest has reclaimed some of its rights (sparse trees and scrub), in which there is a sinkhole and a chasm (marked by old barrier tape on the trees).

(4) Continue eastwards and join a new, wider track that descends southwards for 100 metres. The track joins a new narrow path that heads eastwards through the forest, then quickly through the gigantic bramble bushes. The path is very easy to walk on. Reach the summit of Carsabel (altitude 615 metres). There is no view of the surrounding area, as it is completely obstructed by the surrounding forest. Climbing onto a conveniently placed tree trunk, you can see into the distance...

(5) Leave the Carsabel summit plateau to the east and continue along this comfortable but narrow forest path until you reach a wide but poorly maintained track.

(6) The descent begins here. The track heads south for 100 metres, then south-east for 260 metres, and finally the last 60 metres south-west, until it joins a very wide and well-maintained track (shown as a solid black line on the IGN map, but not on OpenStreet) at an altitude of 520 metres.

(7) Continue the descent, heading southeast, until you reach a perfectly clear meadow, used as a storage area for cut wood, on the side of the road, at a place called Saint-Paulet, in the commune of Saint-Paul-de-Jarrat.

(8) Leave this fenced property on the road side (but not on the mountain side), causing as little disturbance as possible to the owner, and take the road towards the D117 and the locality of Saint-Antoine. You will find yourself back in an urbanised area. At the first crossroads, turn right into the old village of Saint-Paulet and pass behind Saint-Antoine to the north to reach the banks of the Sios, leaving Avenue de Ventrille (which leads to Saint-Antoine and the D117) on your left. You will reach a fork and the Lios riverside path.

(9) Take this charming path on the left bank for 1.3 km, reach Chemin de l'Oratoire, turn right and continue for 100 metres, before returning to the riverside path for the last 70 metres. Reach Rue de la Place in the centre of Montgaillhard. Turn right, cross the bridge, then return to the junction of the loop.

(1) Turn left and head to Place des Écoles (S/E).

Difficulty
I have rated this hike as "Moderate" because, even though the elevation gain and distance are limited, the route through the forest, the almost total lack of markings, and the multiple transitions from trail to path and vice versa make the hike less easy.
It took me two research sessions to find this ideal route.
The IBP Index (automatic route difficulty rating system), a partner of the FFRandonnée, is 38 HKG, which corresponds to an Easy route for a hiker with average physical fitness.

Time
Visorando estimates the time at 2 hours 10 minutes. I completed this route in 2 hours 30 minutes. This is a fairly short time, including a few photo breaks.
Allow between 2 and 3 hours.

Equipment
Bring simple hiking or running gear.

Drinking water and shelter
There is no free drinking water or shelter on the trails, but there are many shelters in the village and you can eat or buy bottled water at the Roucateille campsite.

Variations
There are many variations on the slopes of Carsabel, with some trails marked on the IGN map and others not. But be careful, as some of the marked trails no longer exist or their positions are inaccurate.

Nearby

(S/E) Village of Montgailhard
Pais de Foix Tourist Office
Wikipedia
History of the village of Montgailhard on Histariege
La Roucateille campsite
Le Bon Gaillard Wine Bar

Activities
Les Forges de Pyrène
Visit to the Château de Foix
The town of Foix