Sistem pengaliran yang terdapat di sekeliling Kompleks Bangunan Stadthuys telah dibina pada zaman Inggeris atau era selepasnya. Kompleks Stadthuys yang besar dan gagah ini kelihatan seolah-olah tiada sistem longkang ketika zaman pemerintahan Belanda. Ia hanya dijumpai sewaktu kerja-kerja restorasi dijalankan dan didapati sistem pengaliran dibina melalui ruang bawah struktur Stadthuys. Longkang ini merupakan salah satu dari sistem pengaliran bawah tanah yang dijumpai semasa kerja-kerja menanggalkan lantai simen yang lama.

Terbukti pihak Belanda menyusun dan membina sistem pengaliran sebelum pembinaan bangunan ini dilakukan. Ia menggambarkan corak sebenar seni bina Belanda. Longkang yang ada tidak berfungsi lagi akibat pembinaan tambahan kepada struktur asal bangunan.

Blok-blok batu lada disusun dan diikat menggunakan plaster kapur untuk membentuk sistem pengaliran. Ia kemudiannya ditutup dengan ubin granit dan dilekat kuat dengan tanah liat. Bahagian kecil longkang ini sengaja didedahkan bagi menunjukkan sistem pengaliran yang terdapat di dalam Kompleks Stadthuys ini.

The drainage system that could be seen around the Stadthuys Complex now was either built during the British era or later. This huge Stadthuys Complex seems did not have a drainage system at all during the Dutch. The drainage criss-crossing system was found during the restoration work of the Stadthuys Complex. This is one of those underground drains that were accidentally found while removing the cement floor.

Obviously the Dutch arranged and built the drainage system before the construction of the Stadthuys. This was the typical Dutch style and the method of architecture and construction. This drain is no more functioning now due to the construction of an additional structure.

Laterite blocks were lined up and cemented with lime plaster to form the drain. Then the drain was covered with granite slabs and tightly held together by clay. This part of the drain was purposely exposed to show the drainage system that existed in this building.