This restaurant is a bit out of the town centre, but still an easy 10 minute walk over the river from the main square. (If you're driving there's a big car park outside.) Central Castres, to us at any rate, did not appear to be over-blessed with good restaurants so we were fortunate that this one was recommended to us by the proprietor of the Hotel Renaissance and we ate there two evenings out of three. Brasserie Marso is a well run bistro with welcoming, efficient staff and traditional French brasserie decor on two floors. You may need to book at busier times of the year (05 63 35 79 30).
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