Yes Berlin is a name of Slavic origin (meaning "little swamp"). There was also a Slavic town there.
Or even several Slavic settlements in area of today's city.
Several of Berlin's districts - such as Pankow, Kopenick, etc. - also have names of Slavic origin.
taking into consideration that population of a walled town during that time must have consisted of around at least a thousand people
I would say that an average Early Medieval Slavic town (grod) probably had closer to ca. 600 inhabitants.
Something like this was probably an average, small town or "stronghold" (grod / gard / grad):
But as you say, there was also number of cities which were much bigger - several thousand inhabitants each.
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Some of the most important cities (main grods) of Polabian and Sorbian Slavs in Eastern Germany were:
Original Slavic name / German name:
- Hawolin / Havelberg (main god worshipped by this city: Yarovit); constructed in the 9th century; captured by Crusaders in 1147
Hawolin was capital of Neletice tribe (35)
- Brennabor / Brandenburg; founded in the 8th century, surrounded by rampart in the 9th century; captured by Crusaders in 1157
Brennabor was capital of Havolane aka Hevelli (4)
- Arkona / Arkona (main god worshipped: Svetovid); surrounded by mighty rampart and moat in the 10th century; captured by Danes+Saxons+Pomeranians in 1168
Arkona was capital (and one of two main grods) of the Rujani (23)
- Raciborz / Ratzeburg; constructed in late 10th century along two main trade routes (Bardowick-Wagria and Hamburg-Mechlin); captured by Saxons in 1140
Raciborz was capital of the Polabi (36)
- Mechlin / Mecklenburg; fortified town since the 9th century (open settlement existed already before that), along trade routes from Elbe River to Baltic Sea and from Hamburg to mouth of Oder River; important centre of slave trade (those slaves were mostly Danish people captured by Slavic pirates) in the 12th century; abandoned and burned by its own inhabitants in 1160
Mechlin was capital (and together with Zwierzyn one of two main cities) of the Reregi (37)
- Zwierzyn or Swarzyn / Schwerin; (main god: Svarog) mentioned as Zuarin in 1018, existed at least since the 10th century; captured by Saxons under the Henry the Lion i 1160
- Starigard / Oldenburg in Holstein; fortified town since the 9th century (non-fortified Slavic settlement in this place existed already since the 7th century), Adam of Bremen in 1076 described it in Latin: "Aldinborg civitas magna Sclavorum, qui Waigri di****ur, sita est iuxta mare, quod Balticum sive Barbarum dicitur, itinere die ab Hammaburg" ("Starigard a great city of Slavs, called Wagri, which is located near the sea called Baltic or Barbarian, one day travel from Hamburg").
Starigard was capital of Wagri (11). Wagri were old enemies of Saxons. In 798 in the battle of River Sventana (German name: Schwentine) Slavic Wagri allied with Franks defeated Nordalbingians (Northern Saxons).
- Pozdawilk / Pasewalk; first capital of Wkrzanie (or Ukranie), Latinized name: Vuucri (5)
- Przęcław / Prenzlau; second capital of Vuucri (5), located at Lake Ueckersee. Its "golden age" was during the 12th century, but not much is known about it. River Uecker is the German name for River Wkra or Ukra, named after the Slavic Vuucri.
- Malchów / Malchow; constructed in the 10th century; captured in the 2nd half of the 12th century
- Radogoszcz (Retra) / Radigast (Rethra); main god worshipped: Svarozic, also known as Radogost (he was son of Svarog); besieged by Germans in 1068 but siege was lifted; destroyed in ca. 1128 by forces of Emperor Lothar III, exact location of Radogoszcz is unknown (some archaeologists and historians identify it with remains of a Slavic fortified town found in Gross Raden at the Binnensee Lake; some others point at other locations)
Retra was capital of the Redarii (41)
- Roztok / Rostock; located near the Baltic Sea. Described by Adam of Bremen and Helmold of Bosau. Constructed in the 10th century. In the 1120s - after a siege which lasted for 5 weeks (which testifies to size and powerful fortifications of Roztok) captured by Obotrite Prince Sventipolk (son of Henry the Nakonid, who was son of Saint Gottschalk). After that the city declined and became partially depopulated during the 2nd half of the 12th century (perhaps in the 1160s), but later it started to develop again following the charter on Luebeck Recht by King Valdemar I of Denmark on 24 June 1214 and the influx of German settlers (it's quite confusing to me why Danish Kings usually brought German settlers to their cities, and not Danish settlers - perhaps Danish people were mostly rural and in general not very numerous at that time).
Roztok was one of cities (probably capital )of the Chyżyni - Chizzini (40)
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And several more cities - maybe I will mention them later.
Slavic cities such as Wolin and Szczecin (Stettin) already belonged to Pomeranian - not Polabian - tribes.
Slavic Szczecin and Wolin were described by sources as very big (both had at least 5,000 and perhaps up to 10,000 people).
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Map showing location of tribes mentioned above (with numbers and Latinized names):
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Slavic peoples mentioned by "Bavarian Geographer" (written in Regensburg in ca. 845) and identified by historians as probably living in this area:
Plus number of "civitates" (see below) each of them had according to that document:
1 (Nortabtrezi = Obotrites) - 53 "civitates"
2 (Uuilci) - 95 "civitates" and 3 "regiones" (95 fortified towns and 3 provinces ???)
3 (Linaa) - 7
4 to 6 (Bethenici & Smeldingon & Morizani) - 11
7 (Hehfeldi) - 8
8 (Surbi) - 50
9 (Talaminzi) - 13
10 (Beheimare) - 15
11 (Marharii) - 40
14 (Osterabtrezi = probably "eastern Obotrites" = Veleti - it was a union of tribes, not one tribe) - 100
15 (Miloxi) - 67
48 (Uuislane) - ???
49 (Sleenzane) - 15
50 (Lunsizi) - 30
51 (Dadosesani) - 20
52 (Milzane) - 30
53 (Besunzane) - 2
54 (Uerizane) - 10
55 (Fraganeo) - 40
56 (Lupiglaa) - 30
57 (Opolini) - 20
58 (Golensizi) - 5
Total: 661+ "civitates" (but whether it means fortified towns & strongholds / forts or some territorial divisions, or population units, is uncertain).
This is for territories of Eastern Germany + western strip of Poland + Bohemia.
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And here is the original text of "Bavarian Geographer":
Note that this document lists only tribes about which Imperial spies - "military intelligence" of the HRE - had information. And this is the reason why most of tribes listed by this document lived near the borders of the HRE. Simply Imperial spies had not long enough "range" and didn't know much about peoples living in Slavic areas located further east (for example German spies in 845 AD failed to identify the tribe of Polans, who 100 years later founded Poland):
Descriptio civitatum et regionum ad septentrionalem plagam Danubii:
(1) Isti sunt qui propinquiores resident finibus Danaorum, quos vocant Nortabtrezi, ubi regio, in qua sunt civitates LIII per duces suos partite.
(2) Uuilci, in qua civitates XCV et regiones IIII.
(3) Linaa est populus, qui habet civitates VII.
(4-6) Prope illis resident, quos vocant Bethenici et Smeldingon et Morizani, qui habent civitates XI.
(7) Iuxta illos sunt, qui vocantur Hehfeldi, qui habent civitates VIII.
(8) Iuxta illos est regio, que vocatur Surbi, in qua regione plures sunt, que habent civitates L.
(9) Iuxta illos sunt quos vocant Talaminzi, qui habent civitates XIII.
(10) Beheimare, in qua sunt civitates XV.
(11) Marharii habent civitates XL.
(12) Uulgarii regio est inmensa et populus multus habens civitates V, eo quod mutitudo magna ex eis sit et non sit eis opus civitates habere.
(13) Est populus quem vocant Merehanos, ipsi habent civitates XXX.
Iste sunt regiones, que terminant in finibus nostris.
Isti sunt, qui iuxta istorum fines resident:
(14) Osterabtrezi, in qua civitates plus quam C sunt.
(15) Miloxi, in qua civitates LXVII.
(16) Phesnuzi habent civitates LXX.
(17) Thadesi plus quam CC urbes habent.
(18) Glopeani, in qua civitates CCCC aut eo amplius.
(19) Zuireani habent civitates CCCXXV.
(20) Busani habent civitates CCXXXI.
(21) Sittici regio inmensa populis et urbibus munitissimis.
(22) Stadici, in qua civitates DXVI populousque infinitus.
(23) Sebbirozi habent civitates XC.
(24) Unlizi populus multus civitates CCCCXVIII.
(25) Neriuani habent civitates LXXVIII.
(26) Attorozi habent civitates CXLVIII, populus ferocissimus.
(27) Eptaradici habent civitates CCLXIII.
(28) Uuilerozi habent civitates CLXXX.
(29) Zabrozi habent civitates CCXII.
(30) Znetalici habent civitates LXXIIII.
(31) Aturezani habent civitates CIIII.
(32) Chozirozi habent civitates CCL.
(33) Lendizi habent civitates XCVIII.
(34) Thafnezi habent civitates CCLVII.
(35) Zeriuani, quod tantum est regnum, ut ex eo cuncte genetes Sclauorum exorte sint et originem, sicut affirmant, ducant.
(36) Prissani civitates LXX.
(37) Uelunzani civitates LXX.
(38) Bruzi plus est undique quam de Enisa ad Rhenum
(39) Uuizunbeire
(40) Caziri civitates C.
(41) Ruzzi.
(42) Forsderen.
(43) Liudi.
(44) Fresiti.
(45) Serauici.
(46) Lucolane.
(47) Ungare.
(48) Uuislane.
(49) Sleenzane civitates XV.
(50) Lunsizi civitates XXX.
(51) Dadosesani civitates XX.
(52) Milzane civitates XXX.
(53) Besunzane civitates II.
(54) Uerizane civitates X.
(55) Fraganeo civitates XL.
(56) Lupiglaa civitates XXX.
(57) Opolini civitates XX.
(58) Golensizi civitates V.
In many cases information about number of "civitates" is missing - this of course also due to lack of information (just like many tribes are not listed).
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It is not 100% clear what exactly "civitates" in this document mean (see above).
But undoubtedly number of "civitates" listed for each tribe in "Bavarian Geographer" indicate the strength of each tribe.
Something similar in Anglo-Saxon Britain was Tribal Hidage, which listed 35 Anglo-Saxon tribes and number of "hides" for each tribe:
http://en.wikipedia.org/wiki/Tribal_Hidage
West Saxons was the strongest of all 35 tribes, with 100 thousand (!) hides. The weakest tribe had only 300 hides.